When it comes to your credit, what you don't know can hurt you. And according to a new report, millions of consumers have potentially damaging mistakes on their credit reports.

The report from the Federal Trade Commission looked at 3,000 credit reports. It concluded up to 42 million Americans, about one in five with a credit history, have errors on their reports. The study found that for 5% of consumers -- around 10 million people -- those mistakes are serious enough to result in higher costs for things like insurance and car loans.

So, how do you know if your credit report is accurate, and how do you fix mistakes if you find them?

The essential first step is to go to annualcreditreport.com and get copies of all three of your credit reports. You can do this once a year for free. If you stagger them, you can get a free copy of your credit report every four months.

The bad news is, not very many people do it. But it's easy -- I went through the process this morning and had printed copies of all three of my reports in less than 10 minutes. Annualcreditreport.com is the only place to get a truly free credit report. Other sites' "free" reports come with strings attached, like credit monitoring for which you'll have to pay.

Once you have your reports, check for potential issues. Look for a section near the top marked "Potentially Negative Information" or similar wording. (All three of my credit reports came up clean, but only Equifax's report gave me the "Potentially Negative Information" section anyway. The other two, from Experian and TransUnion, listed "Accounts in Good Standing" or "Satisfactory Accounts.")

If you find a mistake, it can be difficult to fix (as reported in detail by 60 Minutes.) In the FTC study, consumers who found mistakes were urged to dispute them through the process spelled out by the Fair Credit Reporting Act. The results? One in five people had an error corrected by a credit reporting agency after disputing it, and about 13% of consumers saw a change in their credit score from a mistake being fixed. But the change was significant enough to result in better credit offers for only about 2% of participants in the study.

If disputing a credit report error with the credit bureaus doesn't get you anywhere, you can contact your state attorney general, the FTC, or the Consumer Financial Protection Bureau.

Remember, though, you'll never know if your credit reports have mistakes unless you check them.