One rule of thumb that helps me when I'm thrown in a playing situation where I don't know any of the songs is to try and pick the bass note out. Think about it.. what's easier to decipher with your ear? A 5 to 6 note chord or a 1 note bass line? The 1 note bass line wins out every time.
From there that 1 bass note can tell you alot IF you know your theory! Example.. we're in the key of Db major.. the bass goes.. Gb, Ab, Bb.. now since I know the number system and the major scale. Gb is the 4, Ab is the 5, Bb is the 6.. this then tells me that the majority of the time the chords will then be Gb major, Ab major, Bb minor then I'd add my 7th, 9th, 11th and 13ths to spice things up but there you go. I figured out the whole progression just by knowing the key and bass line. I know this is a very simple example but I hope it's effective.
Most musicians will take some time out of playing to tell you what key you're in if they are cool.. lol...
HTH,
DaNatiMaestro