Are you talking watermelons? Male to female flower distinction is very easy on watermelons. Here is a pic of a female flower. Notice the small melon behing the flower? A male will not have that. The tiny melon behind the flower is the ovary. Once pollinated the the pollen travels into the ovary and the flower will dry up and fall off (this one was hand pollinated 2 days before the pic was taken) and the melon will grow rapidly.
Hope this helps? Google or youtube "hand pollination". I did and learned a ton very quickly. Other species of melons, sex identification of flowers can be very difficult.