I've been playing melody instruments for the last 40 years & recently switched to English concertina. And no, I cannot work out chords with any degree of confidence - I usually try filling in what I hear as the bassline & then seeing what happens if I fit the 'bass' note into a major triad. If none of the major triads sound right I try a minor; if none of them works I try sus4 chords. (I use far more sus4s than I do 7s; maybe it's a concertina thing.) If the sus chords don't work either, I give up.

So yes, having chords written down can save quite a bit of work! There are lots of beginners on this board, where 'beginner' means 'someone who doesn't find something as easy as you do'.