Counting is easiest, and very easy. You just need to learn that there are two semitones between all the natural keys, except B to C and E to F, which are just one semitone apart, and you're.

Just look at the key shape you are fingering if it was down at the far end and count up the neck to where you are playing it and that'll tell you what you are actually playing.

Or do the same to decide where to put the capo so tha the chord shape yo want to play will sound right in some other key.

Same applies when you want to play barre chords up the neck, ot chords with some open strings.