Put the top down and open both doors. Then get in and see how the doors line up when you go to close them, you need to be sitting in it though or you won't know if it is flexing without having weight in it. If the door opens and closes good with no dragging or hanging up with you in the car while the top is down and the doors open then I would buy it. But if the door hangs up or binds when you try to close them, then you have more structural rust and I would walk away unless you're ready for the work involved.
