First, it's important to know the exact species. P. hongkongensis are nearly impossible to sex, while some of the other warty species are relatively easy.
Start here: http://www.caudata.org/cc/articles/sexing.shtml
Leo, you are asking a lot of questions that are in the FAQ. No offense, but you might save everyone (including yourself) some trouble if you read them: http://www.caudata.org/cc/faq/faq.shtml