How long to stare depends. Learn the common tells. Here is my method.
Quick Glance #1: Identify watch.
Note: (Is it a common uncomplicated model or complicated chrono)
IF Common Model like a Rolex Sub
Quick Glance #2: Date Mag / Crystal Rise & Rehaut depth.
Note: (This will easily eliminate the low end and Canal Street stuff)
Quick Glance #3: If still unsure, note Sweep, Crown, Crown Guards, Pearl.
Note: (Still can't tell, I assume Gen, and note the persons cloths, shoes, etc)
IF Complicated Model like a 7750
Quick Glance #2a: Eliminate the low-end. Is the Chrono Sweep seconds hand moving? Is the correct Seconds @ 6 or 9 sweeping like it should? (Most of the time: Chronos are OFF, so an always running chrono to me is suspect)
Quick Glance #2b: If it is a 9-6-3 subdial w/date. Does it have a sunken datewheel?
from here you really need to know your models, as there is not enough time to keep staring.
NEVER Call a person out. It would be hypocritical. The fact a person has a Canal Street version on just means he/she hasn't found us yet.