kyle you would use rpe when determining what weight to use. you first have to teach them what hard is which i think is the problem for most women (on resistance training) and men when doing squat and deads.
You don't need to ask them what their RPE is, you can just look at them performing the lift.
You don't find that if someone's max lift is 50kg, then 40, 45, 49kg are all perfect and quick, 50kg is perfect and quick, and then 51kg simply doesn't move at all. As they get close to their max they slow down and form starts breaking down.
Thus, if the reps are quick and the form good, then they are not exerting themselves much. If the reps slow and/or form breaks down, then they are exerting themselves strongly.
And you can look at their faces and responses, too.
Remember also that people have up days and down days. What's easy for someone today will be hard for them tomorrow. So the coach/trainer has to adjust according to the person's performance on the day.
Again, it'd be nice if we could put neat numbers to things, plan everything out. But people are just too complicated for that, we have to go by feel, adjust as we go. Or the plans we make have to err on the side of conservative, allowing for those down days.