Once a year, it's a good idea to check your credit report. You can get one from each of the three reporting credit agencies--Equifax, TransUnion, and Experian--by going through the Annual Credit Report site.
There are many places that say they can give you a free credit report, but the link above will take you to the official place.
You can either do it online, print out a PDF and send it through the mail, or call and request one. You can get one once a year--or, if you want to spread it out, you can set a schedule and have one of the three done every 4 months.