There are so many here better qualifiied to answer than me but I'll try to get started. I like to look at every inch of the frame. I wear my "mechanic clothes" and crawl all over the vehicle I'm looking at. Take your time. Don't be distracted by the things you like about the vehicle look for things you won't. Frame I have seen crack are near the spring mounts for the front springs. Also look for braces around the front and rear spring hangers. Try to get an idea of where it came from originally. Check all the body mounts. If its been bumped a bit the rear body mounts usually show it. Hopefully some of the more experienced guys will help me out here.