I guess we misunderstood your post.
If guys are bringing you their 2nd and 3rd planes for a test flight or preflight... well it's nice to be trusted. perhaps they didn't ask anybody else.
Try this... "tell ya what, I've been watchin you. You're a good pilot. test fly it yourself. However I'll stand next to ya in case theres a problem. Is that ok?"
Now if all is good you can watch and coach, and as soon as he's down you can go with a pat on his back and a "nicely done"