What to ask...

So we're under contract to buy a home and instead of hiring a "home inspector", we're opting for a licensed pro in plumbing (and electrical and a structural engineer). I just saw a post here where the OP was asking for feedback on how he did. The comments included replies that affirm what I already knew, I know jack shit about plumbing.

What questions should I ask the inspecting plumber about the house to sort of test his proficiency and knowledge? What should he be checking? The main area of concern is the remodeled master bath which has the plumbing pretty much open for inspection from the basement. That and the exterior spigots because I doubt the elderly owners were performing proper winterizing steps.