If you can get vent shots, it helps if you have two people. One to hold the fish upside down in a net and another to take a picture.
Here is a good link about it.
http://www.fishhead.com/articles/ventsex.htm
EDIT: I can see the texas's vent in the picture. He looks like a male, because his is narrow and pointed. Females are generally larger and blunt/rounded. You planning on breeding red texs or something?