Its not too difficult if the animals are adult, or near adult sized. Females have VERY short tails. Basically it appears as if the cloaca occupies the whole tail region with just a bit of a tip past the cloaca. With males, the tails are longer and thicker with much more lenght and taper from the cloacal opening to the tip of the tail. However, with babies and subadults this does not always hold true. In addition, male redfoots can also get peanut shaped shells when looked at from top view. Their shells pinch in a bit at the sides in the middle while the female retain more rounded shells in general.