If you can't do another rep, then you went to failure. How do you know? That comes with experience b/c you just have to know where your limit is by feel, unless you want to get stuck under a few hundred pounds. In other words you stop BEFORE you attempt that rep that you can't do.
Now, with curls and pulldowns and the like you can try the next rep and just put the weight down if you can't complete it.
With bench and squat you just have to know (unless you have spot)
I lift alone and have ben liftin for years so I know, sure sometimes I'm not sure and rack the weight 1 rep shy of failure, but that's still plenty intense.
BTW, don't use collars if you lift alone.