I can never tell if someone is riding fixed unless I get close or if I see them freewheel.
Anyway, the nodding/waving thing is difficult isn't it? I see a roadie, I wave, maybe she/he waves back. Maybe not. Mostly they wave, I'm not bothered if they don't. I see an MTBer, I wave. Fair enough. I see an old guy on a BSO. I wave, but I'm not sure. I see an old woman on a shopper, cycling to the hairdresser. On the pavement. Maybe I wave. I see a 14 year old girl on a BMX? A courier? a really serious looking tester when I'm riding ms. P's bike (with basket attached) to the shops? A whole pack of Belgian roadies and you're riding a Brompton?
F***. It's not easy.