I just assume the lower old leaves dry up and drop off when the plant matures. I feel like that’s true in some cases, and maybe with others it’s something else. So hard to know with different varieties having different needs and whatnot.
One of my watering methods is to determine the weight of a dry, thirsty plant vs a well watered one. This way is almost foolproof for me, may take a bit of time to familiarize yourself if you have many plants but worth it!