The sooner you catch identity theft the better. The couple we interviewed had no idea until they went to buy a home - and then surprise! You should check your report at least once a year. Go to annualcreditreport.com for your yearly FREE report.
A couple of years ago, I pulled mine - and I'm glad I did! An ex-boyfriend's name was on it with a credit card HE had opened up! It turns out (unknown to me) that he had opened it up in both of our names. It wasn't negatively affecting my credit, but I sure didn't want it on there! I filed a complaint with the credit reporting agency, and it's no longer on there.
If you need to dispute something, I've linked the Federal Trade Commission's tips. You'll also notice they have a sample dispute letter, which will give you an idea of how to write to them.