Is AD compliance ever physically verified when maintaining an aircraft?
I can only speak for commercial aviation, but in my experience, we generally have 100% RII buyback on initial AD compliance issues. For example, if we have to run a new wire bundle or reroute a wire bundle as part of an AD, it has to be "bought off" by either a QC inspector or an RII-qualified mechanic. In these situations, I would say that the work is done properly 99% of the time. Rarely would someone put their name on something that's done incorrectly if they don't have to go back and do the work themselves. The response is generally "Call me when you get it right." 

Where I think the FAA severely lacks oversight, in my opinion, is future compliance in upholding the standards of the AD. On the initial AD, you're not allowed to deviate one iota from the original without a very special process put in place. But what happens when you deviate 10, 15, or 20 years down the road?

Hypothetical scenario: An AD is written to reroute a wire bundle from other wire bundles because of some safety issue. The AD is complied with and life goes on. Twenty years later, a mechanic disassembles all the wiring in the area for "X" maintenance issue. He turns it over to the next shift to reassemble it. Next shift has no idea how it goes back together. It's not marked as AD wiring. There are no instructions on how it all goes back together (save for the AD paperwork from 20 years ago), so the mechanics put it back together per the standard wiring practices manual. They bundle and clamp. Bundle and clamp. They do a beautiful job, but guess what? It violates the purpose of the AD from 20 years prior. Do you think the FAA follows up on that? Nope. The company? Nah. They have paperwork showing the AD is complied with. So it's up to each mechanic to determine if what they're working on is part of an AD (even one from 20 years ago), despite no special labeling or instructions, and maintain that AD compliance. As we mechanics joke, "It'll all come out in the investigation." 

My experience is that as time goes by, the integrity of many AD's slowly dissolves. That being said, if we as A&amp;P's even smell the faintest whiff of an AD, our antennae perk up and we are very careful to maintain that compliance. AD  violations are the surest way to get a nice letter in the mail from Mr. FAA.