Look for a flap between the booklungs ion the inside of a molt. If there isn't a flap, you have a male. I don't have a good picture of any of the tarnantula molts that I have to illustrate. Here's a link to a site that has some detailed information.Originally Posted by Shelby
http://homepage.ntlworld.com/the.tar.../howto-sex.htm
There is another method, called ventral sexing, but it's more difficult and takes some practice to get the hang off. Here's a link to an article about ventral sexing by Rick West.
http://www.birdspiders.com/faq_sex.html