Sort of, no, not really at all kind of thing.
In some newts, but mainly frogs and toads, the males will grow what are called nuptial pads during the breeding season. Due to hormonal changes, patches of skin on the inside of the forearms will turn darker in color, and become rough (for an amphibian) in order to help aid the frogs or newts in amplexus.
In an answer to your question though, fire belly newts will not have nuptial pads.
Your best bet to sex the newts (if they are adults) is to ask the shop if they can help you to look at the cloacal opening of a few different newts. Generally, in mature animals, you'll see the males cloaca is much larger (particularly during breeding season) while the females is a bit smaller. Also, females generally have a more rounded body than males, but you may just be picking out a fat male, you never can be sure. Good luck!
Don't forget to search through caudata culture to find more information on sexing, and more informational particulars about your newts -
www.caudata.org/cc
