We check the wall underneath before we talk about stone, because veneer is only as good as what it's stuck to. I look for cracks, loose block, peeling paint, hollow spots and soft stucco, and I find out what's under the surface. If the wall has a problem, I'd rather tell you and fix it than cover it with stone that could come loose later.
On a house wall, the water barrier goes on before any stone does. Water gets behind any wall finish sooner or later, and the barrier, lapped the right way, carries it back out. You won't see that work once the stone is up, so your estimate lists it, and you can check it against the two layers covered above.
Sometimes the honest answer is that the wall needs rebuilding first. A cracked or leaning yard wall may be better replaced with new [concrete block walls], and a failing base under a house wall may need [foundation block wall installation] before any stone goes near it. A new block wall gives the stone a solid, clean face to bond to, so you can have it faced once it's built.