You COULD get yourself a pigeon, they are able to discern the difference between a watercolour and a pastel so telling a watercolour from a print should be a cinch I imagine
http://www.telegraph....ood-art-from-bad.html ;-)
...but seriously though, googling:
watercolour or print "tell the difference"
...will render you a multitude of threads. My best bet would be to look at the texture of the paper but I'm no expert. Still if it's watercolour paper it's always at least slightly grainy isn't it? And thickish. But it can be tricky, obviously, and I don't think you could ever be sure, as an amateur.