What you're looking at in turtles is not tail length itself, but precloacal tail length. In a female the cloaca is right where the tail joins the body. In a male there is a thick section of tail between the body and the cloaca; the male's cloaca is usually about even with the edge of the carapace. This works for softshells and all other turtle species I am familiar with; I don't know about sea turtles, tortoises, or pleurodires.