73 classic and 73NG - doesn't work if you're looking at a photo, only if it's live, but the NG has synchronised strobe beacons on top and bottom of the fuse - i.e. they flash together. The classics aren't synchronised so don't flash together.
Main diff A319 to A320 is the overwing exit as already mentioned, except for easyJet. I guess with more low costs taking A319s, though, that may well change and more will appear with double overwings.