Um, the taste? ;)
Seriously, semen should be more liquidy, but I suppose this could be difficult to figure out. Semen smells kind of ammonia-y, too but I really wouldn't want to be smelling my fingers after fishing around in there.
Good luck figuring this one out!
semen has a very distinct smell, smell it next time you are in contact. that is pretty much the only way to differentiate between the two, smell and taste.