You assume your credit score is solid. After all, you pay your bills on time, spend just a fraction of the limit on your credit cards, and take all the other steps required for a clean credit history. On top of that, your last landlord ran your credit and told you it was pristine.

But you may be in for a shock the next time you try to buy a car, or sign a new lease.

“A lot of times, people don’t know what they don’t know about their credit history,” says Barbara O’Neill, Ph.D., a professor of financial resource management at Rutgers University.

Every year, roughly 1 in 50 Americans falls victim to identity theft, O’Neill says. “For most people, it’s not a question of if they’ll have their identity stolen, but when,” she says.

If someone’s swiped yours, the thief could be opening up new credit cards in your name, charging purchases to your existing cards, or even posing as you to snag free medical care, she explains.

While keeping close tabs on your credit card accounts will alert you to some of this fraud, in many cases noticing odd activity on your credit report is the only way to know something fishy’s going on, O’Neill says.

Considering the importance of your credit score, credit agency mix-ups and errors are frighteningly common.

“If you have a common name, or you share your name with a family member, comingling of data and information happens all the time,” O’Neill says.

For example: You and your dad share the same name. Because you two used to share an address (when you were living under his roof), it’s not unusual for the credit agencies to confuse the two of you. If senior buys a new house or files for bankruptcy, you may erroneously take the hit for his big purchase or penury, O’Neill explains.

You may also be confused with a total stranger who shares your name, she says. And sometimes the agencies just screw up and ding you due to a computer or reporting error.

There are three major credit-reporting agencies that compile and score your credit history, O’Neill explains. Their names are Experian, TransUnion, and Equifax. Government regulations require each to provide you with a free annual credit report, which you can access via

While there are a lot of sites with similar-sounding names—many of which contain the word “free”—stay away from them, O’Neill says. They’ll either try to charge you fees, or sell your personal info to marketers.

O’Neill recommends checking one of your three credit reports every four months, and alternating among the three reporting agencies. So this month you might pull your Experian report, in four months you’d pull your TransUnion, and four months later you’d check Equifax. Then do it all over again next year.

“It’s still free, since you’re only checking each one once every 365 days,” she says. “But if something goes wrong or your identity is stolen, only a few months will pass—at most—before you’ll find out about it.”

It takes about 15 minutes to download your report and check for errors, she says. “Compared to the time it would take you to fix a problem if a year or two passed before you spotted it, 15 minutes every four months is a pretty small investment for your peace of mind.”