As suggested above most shops will have the right kit to be able to tell you. However like the picture above shows you can do it with string too.
Loop string around the headtube through both of the rear dropouts and back to the headtube, and tie it nice and tight. You want to look at the distance between the ST outer edge and the string on both sides, use a ruler/vernier to be sure. You want them to be the same on both sides to know everything is still straight.
If it isn't then it's either the front or rear triangle that will be out (or both possibly). LBS will have rear dropout realignment tools that allow you to see how parallel the drops are and can be used to bend them back into place and in conjunction with a frame alignment tool to get everything where it should be.
Also look at how the wheels are sitting in the drops and if the brakes and rims/discs are still lined up. Check the forks for any damage to the steerer too incase it has been crashed head on.
Good luck, and i hope my explanation isn't too shabby.