The quickest and easiest way to get a handle on how dry your filament is is to drop the spool into a poly box like these with a hygrometer, close it up, and let it sit a couple of hours before reading the hygrometer. No desiccant. Just spool and hygrometer and also know best results will be with full to mostly full spools. Too little filament or if these are cardboard spools can throw the results.
The humidity value it stabilizes to is in effect a measure of moisture in the filament but is modified by filament type. The numbers won’t mean the exact same so if you start doing this only compare numbers of similar filaments. PLA to PLA, PETG to PETG, etc. (Added - it could be they all behave pretty much the same too but no data yet to support saying you can compare across filament types. We just don’t know yet.)
A properly dried spool should cause the humidity to plummet. On my filament they all go to 10% RH pretty quick. That has been good for all my printing. Any numbers above that and there’s more water in the filament. Undried PETG HF has had numbers like 40 and 42%. After drying, 10% or less. (I don’t know the actual humidity when they display 10% - just that it’s 10% or below since that’s as low as my hygrometers go.)
Something to also keep in mind is we don’t have a good handle on how low is necessary. It could be 20% is good enough, but if you see numbers above 30% it’s probably getting iffy. Anything higher is worse. But I don’t see moisture effects now with numbers at 10% or less.