Caporegime
Oh and as for spotting, you can tell when someone is having real difficulty. Like when one side of the weights starts dipping and so on. Like UE says, it's also usually at the 'low' end where trouble is likely to start, which is also the least likely to cause real harm. I imagine that people watch the power lifting videos where these guys are really, really pushing their absolute limits of 1 rep max lifts where it can go wrong in the blink of an eye, but for the everyday workout of 8-12 reps it's not that sudden.