I've always been successful with my "baseline" comparisons. That is, with weekly washes, i know how it behaved right after it was applied. Beaded or sheeted water as good as its going to right after it was freshly applied. I then take note of how it is behaving after each weekly wash and during rinse down. No dirt left on the paint and in its cleanest, hydrophobic state. How's it look?
When i notice the gradual drop off in hydrophobic performance i reapply. Because of that i never am without at least some wax or sealant left but can tell when its going away, getting thin and just barely holding on.
I will say that is also the point in my evaluation (still wet and prior to fully drying) that i evaluate my LSP's life because i always use a QD product that most times adds a little pop and further but short lived protection. At the point or weekly wash that it's time to reapply my wax or sealant i forego the QD and break out the LSP again.
Again, in summary, i know how it looked when newly applied and use my eye to tell me after the wash and rinse if its still there or at least getting thin. Sometimes that's a week or two for waxes and maybe a month or 3 for sealants. Certainly not very scientific but i get a feel and "look" for it.
At the rate this business is coming out with new products in developing more of the science of what we do, i won't be surprised to see some tool, machine or device to measure LSP effectiveness! Maybe some strange sextant looking device to measure water bead angle or LSPTG (LSP thickness gauge). Heck, at $300 a pop, throw in a "special gift" of 2 MF towels and theyd fly off the shelves!!!