If it’s any consolation, I can’t tell when my albino, 2 bananas, or my dragonfly are going to shed either. The light colors just don’t show the dulling very well, until one day you have a shed in the cage.
I would think you could use something like tissue paper or parchment paper (baking supply) under the substrate so that the laser temp doesn’t reflect off the glass. You might have to wait for it to warm up for a few minutes to get an accurate reading if you’re just going to use it when you measure temps.