Assuming these reps are top of the class,
For Rolex you can see across the room the metal shine. Then 5m you can see rehaut. Up close you can see pearl and crownguard.
For Panerai you can see the unmatching crownguard 5m. Dial font size, spacing, and color are dead give aways upclose. Movement is also too easy. Even modded panerais lack in these areas.
For Omega it depends on model but generally most Omegas have HE valve in the wrong place. Then the logo and AR.
The surefire way is to memorize all the rep serial numbers and take a look. Also, sunken datewheels and seconds in the wrong place is a give away too. I've picked out quite a few reps in my short life. But usually I just ignore or say something like "that's a nice ersatz watch you have there" with a smile (ersatz = fake) and they think it's a compliment.