In the past one of the easiest ways to spot real ones from the movement. A Rolex second hand should be smooth as silk and not making a ticking noise.
Like Tone says though, that may have changed more recently with the quality of fakes these days.
EDIT: Quick google suggests I'm wrong - Rolex also make ones that tick!
Genuinely interested so did a little googling. Seems like if it's real everything should be more than millimeter perfect. However, given the suggest age of the watch it's hard to know what's wear and tear vs starting like that.
For example, the 6 o'clock marker is squint, and the text on the watch face isn't perfect. Is that because it's a fake or because it's old? No idea
You have been able to buy clones of high beat movements for many years now. The Swiss Valjoux 7750 is available as an Asian ETA 7750 for instance with the same 28,8000 vph responsible for the "sweeping" second hand. So even that isn't an indication anymore.