Hi - if you are looking to do something yourself at home to get an idea of what you have, before you take it to a shop, you can get a good idea simply with a tape measure and a buddy.
Assuming you have the frame stripped bare, and can see the top, you will find a whole bunch of stamped holes, rivets and bolts. Take the tape, pick marks front and back that are in the same position on both sides of the frame ( a punched hole is usually best ) and measure diagonally to make an "X". Any difference between the measurements will indicate the frame is out of square. ( make sure if you use a hole, decide if you are going to measure from the front lip, or rear lip for consistency on both measurements ).
I have had to do this a couple of times on old frames. Start with the longest distance you can front to back, to see what the overall frame is like. If it is off (more than a couple of mm or 1/8 " ), then break it down to ( 1 ) front bumper mount to front suspension ( 2 ) front suspension to rear suspension, ( 3 ) rear suspension to rear bumper mounts. That should give you a more detailed idea of where the damage is, and how bad. Typically I have found the front frame bent in front of the suspension cross member, and then square after that.
Its not perfect, as the "fit and finish" from OEM was always subject to assembly on the day, however what you are looking for is a "relative" measurement of how square the frame is and therefore if you can use it or not. This method is quick and cheap tho', and can be done at home easily to give you an idea of what you have.
There are frame drawings kicking around the net with measurements also. I am away at the moment working, however I'll see if I can post a couple for you when I get a chance.