OUCH! I thought breeding questions would go in the breeding forum but I guess livebearers are banned!
Anyways, for me the best way to tell the difference between a pregnant fish and nonpregnant fish is how fat they are. When they get really fat than they are about to explode with fry. Look at some pictures of nonpregant fish to compare.
I was wondering how to tell the difference between swordtail females and platy. They look exactly the same to me!