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here's a picture of my male blue ram. the males are more colorful and they have the solid black spot on the side. the female are less colorful and the spot on the side has some color showing through. when i got mine they said to get 1 male and 2 females and they would develop a pair. however they gave me 2 males and 1 female and the female died the next day but i've had the males for 6 months now and they are doing great. i think the female died from the stress of the move to my aquarium. hope this helps.
With Bolivian Red Rams - obviously this might be completely wrong for GBR's - where the tail meets the body is an indicator. Look for overall general roundness. I'm not home ATM so I can't post but one of my females is just plumb, while the male has a more slender body.
Also the males do sometimes have slightly black on their pelvic fins when fully mature.
If they are not mature then the easiest way to tell i think is the pelvic fins, because the female will already have the black line and the male will have no black line at all