Don't think mechanics are stupid. I used to see that crap then change the parts and put the marks back on the new parts. I had NO problem pulling the new plugs or whatever out to show the jerk that owned the vehicle that the parts had in fact been changed when they complained. If you have such a distrust for an establishment, do the work yourself.
Just my .02
Considering it is the first time he has used this shop I think it reasonable and prudent to check out their work on your bike. I don't have a problem with checking it out period as I always go by the motto, "Trust but Verify".