Inspect the edges of the back fender opening very carefully. Lots of bubbles mean lots of rust repair. The most common areas from the top are the bottom of the rear quarter panels behind the tire and the front lower corner of the doors. If it is minor, it is not to expensive. If you see huge holes, it gets spendy. Get under the car and look at the areas where you feet sit when riding. Lots of holes or metal riveted in usually means floor pan replacement. Cheaper than quarter rust as it is not needed to be smooth and pretty.
As was said, pour lots of water in the vents between the hood and the bottom of the windshield. It should run out by your feet just behind the front tire. Have some one look in the interior by the brake pedal on the left side or the place where you feet sit on the right side when you pour the wter with a flashlight. If water comes in the interior, you have cowl repair. It can get spendy.
Engines, trannys, brakes, and suspension are cheap when compared to rust repair. look it over carefully and get under it or get it on a rack. It will tell you alot!!
Rob