Its kinda a guessing game. I find it helpful to keep molt records that helps you dial in somewhat with guessing when a molt "could" be coming. But also in general you can still look for some basic premolt signs. Those would be, refusing food, loss of coloration, webbing their hide or burrow closed.
I use a Led flashlight as well and shine it through the legs, you can see the new exo growing under the old one sometimes, especially at the joints. But on my burrowers thats pretty hard to do when I cant see them.
Record keeping is the best way over all I think.
Kinda the same way as New World, only this takes much more experience.. Its all in the general looks of the spider. 'fur' rubbing off the head, coloration dulling in cuticle, just general 'shabbiness'. And the further from a molt, the more obvious the sign until eventually, you just know there's a molt coming.
Hair on the legs usually rubs off over time, and once the rubbed off parts turn completely black you know they're gonna molt soon. Also, they appear sluggish and/or waisted, but don't be fooled when bothered they will turn around and try to bite the heck out of you. THey basically aren't themselves.