As Americans head to the polls on Tuesday, all 50 states have contacted the Homeland Security Department asking for some form of assistance checking the security of computer networks or voting machines, a department official told The Daily Beast.

Since Russian hackers breached the computers of the Democratic National Committee last summer, federal officials have been practically pleading with states and localities to take the free help the U.S. government offers, including scans of state and local computer networks to check for security weaknesses or signs that hackers may have already gotten in.

This election cycle has drawn more attention than any other to the threat of hackers changing votes and sowing chaos and confusion, and claims of rigged elections—by computerized or other means. Nearly two dozen states have seen their voter registration files probed. And following a major cyber attack last month that disrupted core components of the internet, experts are concerned that such denial of service attacks on Election Day could make it difficult for people to find their polling location or for precincts to transmit voting results.

That anxiety, though, has translated into action, five election security experts said in interviews Monday.

“Systems are secure,” said David Becker, the executive director of the Center for Election Innovation and Research. “This does not mean their impervious to anything,” he cautioned. But broadly speaking, election officials and the public are more aware of and on guard for threats to the election system than ever before, Becker said.

“States have been preparing for this election for two years. The gear is ready. The election workers have been trained,” Merle King, the executive director of the Center for Election Systems at Kennesaw State University, told The Daily Beast. “Although many of the issues surrounding election and voting systems that have made headlines in the past year are new to the public, they are not new to election officials.”

That doesn’t mean election officials are taking Tuesday off. They’ll be watching, as they always do, for the signs of something fishy. Often a problem at a polling place—long lines, say—points to an innocuous cause, like higher-than-expected turnout.

But there are certain red flags that could indicate hackers have managed to compromise voting machines, registration files, and other key components of the election system.

Here are five warning signs that election 2016 may have been hacked.

Unusual Ballot Requests

By law, no one can be denied the right to vote if he believes he’s properly registered. If a voter finds he’s not listed on the rolls or that his address is wrong, he can demand what’s known as a provisional ballot. His vote will still be counted, although state laws differ on when.

However, if precincts report a larger than usual number of people requesting provisional ballots, “That could indicate there’s an issue with the voter rolls,” said Pamela Smith, the president of Verified Voting, a nonprofit group that advocates for transparency and security in U.S. elections.

In such a case, hackers may have deleted names from voter files or changed addresses. This could lead to long lines at polling places, particularly if more provisional ballots have to be ordered.

Electronic Poll Books Fail

In some precincts, those voter registration rolls aren’t kept on paper but are stored in electronic devices. In Washington, D.C., for instance, poll workers use iPads that are synced to the district’s master registration file to check voters in at the polling place. Some, but not all, states link those devices together in a network. If hackers disrupt or disable those networks, they could block the poll workers from accessing the lists.

If election officials hear more than the expected number of reports of malfunctioning poll books, they’ll investigate. If the rolls are inaccessible, officials can use a paper copy or retrieve a backup file, which most states make each night.

Poll books “have been scanned, hardened, and are under continuous monitoring throughout Election Day and Election Night,” King said. “These systems have redundant components and can be quickly recovered if an anomaly occurs.”

Rumors Spread via Social Media

Election officials said they’ll be watching social media for signs of hacking, but they’re more concerned about false rumors than real problems.

Consider one common lie that gets spread around during election season and almost surely will be this year—that people can cast their votes via text message. There’s not a single precinct in the United States where that’s allowed, and claims to the contrary could serve to persuade voters that they don’t need to stand in line to cast their ballots, thus suppressing turnout.

“If a disinformation campaign happens, whether it’s sporadic and individualized or widespread and coordinated, I don’t think there’s any question there will be people who will try to exploit that for their own purposes,” said Becker.

As voters head to the polls, “voter misinformation” such as false instructions spreading on social media are one of the primary areas of concern, said Kay Stimson, the spokeswoman for the National Association of Secretaries of State.

“So far, no red flags have been reported in any state, but the nature of contingency planning for elections is to anticipate that issues can crop up anywhere,” Stimson said.

Election officials are also on watch for claims that vote counts have been manipulated, which could erode confidence in the legitimacy of the outcome. Tweets and Facebook messages suggesting some nefarious shenanigans will be monitored and, when necessary, rebutted, experts said. And it has escaped no one that Donald Trump in particular has insisted that if he loses, it will only be because of a “rigged” election.

“Creating and disseminating rumors and misinformation about polling locations, ballot tabulation, and other election activities could be disruptive,” King said. “Election officials are aware of this potential and are monitoring social media posts.”

Equipment Breakdowns

It stands to reason that if voting machines suddenly stop counting votes, or registering votes for candidates other than the ones whom voters selected, something is amiss. But experts also say the machines in use in many states now are so old that they’re prone to error.

Touchscreen technology on some machines is a decade old. If they’re not properly calibrated, it’s possible a voter could find it harder to physically check the box next to a candidate’s name by pressing on it.

“I would be surprised if we didn’t see more of that than in past years,” Smith said.

Some machines also face problems with straight-party voting, an option that lets someone chose every candidate in one particular party. Say a voter in Ohio selects Hillary Clinton for president and then tells the machine she wants to vote for Republicans in all other races on the ballot. If the machine hasn’t been properly programmed, it might deselect Clinton and register the ballot as having no choice for president and Republicans down the rest of the ticket.

Theoretically, a hacker could get access to the files that program machines on how to respond to voter commands and register a straight-party vote for every candidate but president. But even if he managed to trick a machine into performing counter to the voter’s selection, the voter would be able to see that her final printed ballot didn’t match up with her choices. That is, of course, unless she’s voting in one of five states that don’t use paper backups or receipts, a major security weakness that sends voting experts into fits.

As difficult as it is to hack an individual voting machine, experts will nonetheless be watching for instances of incorrect tabulations. Even if the cause is simple wear and tear on old machines, that could still disrupt the election.

High Numbers of Abstentions

This is a warning sign that could actually trick poll watchers into suspecting a problem that’s not there.

Sometimes, voters don’t select a choice for president and vice president. But when that happens, election officials consider that the voter may have cast their ballot in error, since those races are the most important and usually not ones that people intentionally skip.

Election officials refer to the inability to determine voters’ intent in such cases as the residual vote rate. A high rate usually suggests some mistake in how the ballots were cast or recorded. A spike in that number on Election Day could indicate that the machines aren’t properly counting votes for president.

Or not. In this election, experts expect more people are likely to intentionally not vote for president and vice president than in years past. That’s because Trump and Clinton are the two least popular presidential candidates in the history of polling, and there’s a strong incentive for voters to register a protest by abstaining from the top of the ticket.

“The residual vote rate has evolved into a metric of the accuracy and effectiveness of a voting system,” said King. “In this election, there is expected to be a larger-than-usual number of voters who skip the top two races… This could lead to a false positive regarding the accuracy and security of the voting system.”

In other words, while officials usually see a high residual vote rate as a sign of errors, this time it might actually reflect people’s choices.