Kudos to you for saving what most people wouldn't give a second thought towards. Since it's not an adult your best bet for sexing would be by spermathecae(look for these inside the abdomen of a molt - present in females, absent in males) or epiandrous fusillae(bottom of the abdomen on the actual tarantula). Here are a couple links you may find useful, the second being part of a forum that is loaded with great information. They even have designated areas for posting pictures of your spider so other members can help you determine the sex.

http://www.tarantulas.com/sexing.html
http://www.arachnoboards.com/ab/gall...images.php?c=2


I wouldn't suggest offering roaches that have been in with the spider to other animals since they could have picked up some urticating hairs, I imagine these may be especially irritating to an amphibian.