Biggest tells depend on the model. Here is a general list of common tell signs:
- date wheel font
- hands shape/ length/ overall finesse
- crown height
- any rep of a watch with an in-house movement and an open case-back
- crown direction for setting time and date
- movement and complication limitation
- thickness for some watches (e.g., A7750 AP ROO)
- dial colour
- crystal AR
- the stock sh!tty leather strap
- rotor noise for anything that uses miyota or 2813 to imitate in-house movement
- slow date flips for some models
- incorrect bph
- etc
As for the person wearing the watch, well, you would be surprised at how many flip-flop wearing, invisible millionaires in HK who wears gen rolex