1) Buy from a trusted source. Probably the best way.
2) Look at the serial code/clasp
3) Do research. Many models have minute differences that you just have to learn and compare pictures with. Such as printing, bezel, font, sunburst, dial color, etc.
The last advice doesn't depend on Rolex :
4) Depending on the model there are only certain factories that make the model so it basically is a instant tell. For example, Tudor FXD is only made by 1 factory, M+, so it's very obvious.
Long story short there is no "quick" and "easy" way to know. You have to do your research and be well versed in the replica game.