Well, if you have a non-lsd tranny, look through it where the axles go, if there is something blocking it(a cylindrical bar going across), it doesnt have an lsd. Check that, and if after that you can see directly through it, they put it in. If you have an lsd now and you are just upgrading it, i dont know how you would know if they are ripping you off.
You could diy to know for sure