One thing to think about, if you're in common practice music, most of the accidentals will be there for a specific reason (e.g. chromatic grace notes, applied dominants, harmonic or melodic minor, parallel key borrowing, augmented sixth chord). Music theory classes can help understand these if you're not familiar. But the thing to know is whether the particular reason involves raising or lowering the natural note in the scale.

One thing that frustrates me about music notation is that when you have a key signature with lots of flats or sharps, it's hard for me to tell at a glance what the accidental is doing relative to the key signature. I've gotta look back and remember what was the unmodified note.