I just had my shower rebuilt by a contractor. It's done in 18"
travertine tiles. The tiling was finished about a month ago but we
haven't used the shower yet (more work in other parts of the bathroom
are still underway). It has recently come to my attention that shower
niches are notorious for leaking and that they need to be properly
waterproofed. I would like to ask my contractor if he did this but I
don't know what questions to ask. I've read that the bottom of the
niche should be sloped to promoted drainage (which it is) and that
inside the niche (behind the tile I guess) there should be some sort
of sealing material like RedGard.

Is there anything else I should ask about?

Thanks,
Dave