Finding the Metronome Setting (BPM) of a Song

Start your song and turn on your metronome. Adjust the metronome until it sounds like you're in the ballpark. Stop the metronome and start it again exactly on the first beat of a bar. If it seems fast, slow it down a smidgen, stop it again and start it on the downbeat of the next bar. Repeat this process until you find a tempo that matches the band for around two bars (these are live musicians, you won't get the metronome to stay with the band any longer than that).