When in need of a spot, I always ask the most experienced looking guy in the area. 99% of the time, this concludes in a decent spot, the other 1%, it ends in a shitty spot, and I feel damn stupid for failing to properly identify an experienced guy.
Case in point: about 6 months ago I request a spot from the strongest-looking guy in the group of frat boys who were working in the neighboring bench - slim pickens at 6:15am... dude gives me a lift off and walks away! Thankfully I noticed mid-set, before the spot was needed. Unbelievable.
I am now very articulate in requesting a spot: "hey man, do you mind spotting me? I'd like a solid lift-off, and I am aiming for eight good reps." Funny thing about someone mentioning acting ignorant in the gym, because I do the same thing. I act like I've never trained before in order to disarm people when asking to work in, etc. When you're larger than most, it actually works very well if you are overly humble - please and thank you goes a long way when trying to avoid shitty gym habits from newbies.
I am surprised nobody has mentioned the young guy(s) at every gym that brings his girlfriend with to WATCH him.She doesn't train,just follows him around watching her manlol.Usually the guy is 170-175 lbs with bad form using too much weight.