I usually wait until they start playing.

A mustang produces no visceral response on its own that I can recall.
But back in the day the student model mustangs had a 22.5" scale compared to the non-student model at 24" so lots of people (especially older ones) think of Mustangs as kid guitars and pass judgement on adult players of them.
"teh most gains"
Lots of people? Or a couple of idiots?

This isn't really a relevant topic here, we try to stick with concrete questions directly about the gear. If you want to talk about judgmental responses to people's choice in instrument, try the Pit or Musician Talk or TGP.