Males have large long tails they have way more color and are smaller than the female and don't have that blackspot. They chose the females around...
The females lower back fin (not the tail) is shaped like a fan and for the males this fin is pointed.
I dont go with allusive life because males can have black spots to but what i think she was saying is that right behind there belly is a black spot.This sometimes cannot be seen if your guppy is a full color and not the silver see through kind of color.THe tails part is sometimes true but sometimes not tending on the breed of guppy but the tail is generally bigger.Males also can have black spots if it is part of there natural color.
The best and most positive thig to go with is my first paragraph.Hope it helps evryone!