The best way to tell is to use it quite a few times so you go too far and don't go far enough. Usually you'll feel the pad "grabbing" more if you've gone too far with polishing (also if you apply too much polish to begin with, but let's assume you use the correct amount). If you don't go far enough usually you'll notice that when wiping off the residue you'll need a good 2-4 wipes to actually get it all off because it'll still be "greasy" and stick to the paint.
Short answer in general is what legacy said, that it'll look pretty faint on the paint surface but still visible... sometimes barely on black, gray or white.
