exactly; But, while each fish is a little different (especially with inbreeding becoming more prevalent, as always) but its a pretty good rule to go by. That combined with observing their finnage and behavior can normally give you 90ish% certainty. To be positive pretty much need to vent them. Or breed them. I vented my guy when I pulled him from the tank at the LFS when I bought him just to be sure. He's very much a boy. thanks by the way, he's quite the little fella, I love him dearly, sooooo much personality in that fish. I've been meaning to do another update on him and his tank...he's probably the most fun I've had keeping an individual fish yet.