I discuss what fatigue feels like – the weights feel heavier and/or it takes longer to complete a repetition because they’re getting tired. They might be able to “feel” the muscle that’s working, but it should be a general feeling in the region being worked, as opposed to a sharp or shooting pain like if they stubbed a toe or sat on a tack.
For clients that have really low body awareness, I watch their form closely. One sign of fatigue is deteriorating form; a client will “cheat” in order to recruit other muscles to help them do the movement. If a client has been properly doing an exercise and then starts to cheat, I consider them fatigued and have them cease that exercise.