I actually found a display basket of, like, a dozen large willow balls in an antique shop last summer. They looked just like the ones I buy that cost $4-6 each and get destroyed within a few days. I couldn't tell if these willow balls were treated or not (no smell, not shiny, but a bit of dust on them), but it was a deal at $5 for all of them, so I went ahead and bought 'em.
Once I got home, the rabbits absolutely refused to touch them. They wouldn't go near a single one. Ever. No matter where I put them, they knew they were yucky chew toys!
Disppointed with the reception of my second-hand willow ball purchase, I ponied up the cash and bought a bunch of willow balls and baskets from binkybunny a couple weeks later. When I put those out, the rabbits immediately ran up, sniffed, & started nibbling every one I put out. They knew from a foot away these were expensive chew toys worthy of their attention!
So yeah, I don't know about other peoples' rabbits, but my buns seem to know what to eat and what to avoid -- even if I can't tell a difference.