well there is no real way to identify them unless you have it to of that they have a hack that goes with it that is identifiable
Most hacks, if not all, are identifiable one way or another. The difficulty in spotting these, however, varies. Visual hacks such as ESP and tracers are extremely difficult to spot, but combat and movement hacks are easy to hard depending on the hacker's settings.
NOTE: I am NOT promoting any hacking channels or content below.
All in all, the more you PvP, the more you will learn how to spot hackers. Also, not trying to promote hacks, but watching hacker gameplay (e.g. xTurtle) can supplement your knowledge as to how hackers play and behave in game.