Platy Or Swordtail?

[fee de mer]

I was given a fish by a friend who is switching to marine, he told me it was a 'red wag female swordtail'. My male swordtail did sometimes chase her, when he did maybe it wasn't mating? I never really thought very much of this as i have heard that the males sometimes have favourites.
Last weekend i went to my local fish store and bought a male and a female platy, now i am even more suspicious as she/he keeps chasing the female platy. Also whilst i was there i saw some swordtails who are much bigger than her/him (i hadn't thought much of her size before as my swordtails are still only young).
I looked up sexing fish online and she/he seems to have a gonopium like most male platys.
So is my female swordtail in fact a male platy? Please help.

 

[OldMan47]

If you have a fish with a legitimate gonopodium it is a male. We also have a gender identification thread at the top of this section. It has clear pictures to help you determine your fish's gender.