I kind of do it in reverse, when I'm on the tube, I try to spot my fellow Americans. Style is the dead giveway (if I don't hear them talk).
But, I think that's really it. I haven't noticed actual physical features where I'm like "oh..clearly they are from -------" or anything like that. I mean, one of the guys here looks like your very stereotypical California surfer type, but was born and has lived only in the UK.