Do the string test (see Sheldon Brown) to measure the frame straightness from the headtube back.

If the back end is straight, then first LOOK at the forks to see if they are bent. Typically, forks bend back in a front end crash, so stand on one side of the bike and see if the top 6-10 inches of the fork leg is at the same angle as the head tube. Now look at the bike straight on and see if the fork blades are pushed over to one side or the other.

If the fork is bent, you can bend them back if they are not too bad, but you will likely need special tools.