Hey Kirk, there really is no sure fire way but I have a way that is not really recommended but works for me on my personal cars. The safe way is to trust your gut feeling if you think you need to reapply sealant. The way that I do it is: I get my Wipe Out Surface Cleanser Spray and spray it on a very, very inconspicuous area of my paint and wipe it out. I then compare the area that I have stripped clean to the rest of the finish. If the cleaned spot feel less slick or looks duller, there is most likely still sealant on the car. Again, I don't recommend this way at all but I just thought I'd share my personal way of doing it.