If you are looking at a car and are unshure of the amount of body filler beneath the paint ,this trick works well . Get a rubber  refrigerator magnet place it on the areas in question. The low power of the magnet will not stick where excessive body filler is used. In the picture we are showing a magnet over a body filler skim coat.
This is one trick i learned of one of those build a car in a week shows, and it works very well. On this 1972 Chevelle there is a body line where the tape is located. this panel has a mild wave in it and needed a skim coat of filler to make it perfect. The tape gives you a edge to sand to , and leaves a nice crisp line.
A closeup of the tape guide
Here is where the final quality of the finish begins. After applying a coat of high build primer, apply a light coat of SEM GUIDE COAT as seen on the sem products line of our website. By block sanding this guide coat off, your highs and lows will become aparent. This is the method all the top body shops use for show winning finishes