This is where it’s important to listen carefully to what is being played and then try to find that same chord on the guitar. The chord finder here is very handy for that. 

When I am roughing in a song by ear, I recognize the chords by playing the notes. That gives me a quick and dirty way to write down my chord changes. Example would be A and this is a recent example of what happened.

 I knew the song changed to an A chord, because the A note fit over that section, but A major didn’t work, A minor didn’t work, nor did a power chord.

I opened the chord finder and started trying different A chords over that section of the song and it turned out to be an Asus4. I had never heard of it before, but now I can play it.

So some leg work was involved but I did get it.

Hope that helps.