I usually do 2 passes. If my blade is sharp, then that will give me a DFS shave and that's what I aim for. I might look around and see if there are some patches left, but generally there isn't.
I'm pretty BBS after two passes but need that third pass. I use to do 4 passes but after knowing my technique was getting better I didn't need the extra pass.
It's kind of a combination of things... mostly touch. When I'm rinsing between passes I'll feel around for spots that need more attention on my next pass. Also by sound, and the biggest decider on when I call it "good enough" for the day is if I'm on the edge of irritation (or just over...) and I've already finished my regular passes and a few attempts at my trouble spots.
I'm pretty compulsive and I will keep shaving until there is no stubble to be felt. Sometimes that can be a bit much, but usually I get a bbs shave with no irritation.
Tough choice. I use touch, site, and sound. My neck is done when it looks presentable. I use sound and touch for my sideburns, and my mustache is touch and site.
If there's hair on the blade, I'm not done. But if all I'm getting is a layer of dead skin and if all my razor is doing is exfoliation...then I'm done.
I do my usual routine. I can tell if my technique was right on when I'm applying the ASB. Unless I missed a spot completely, touch ups usually aren't necessary.
I'm done when the voices in my head stop commanding me to do more passes.
I voted other: I can feel how much hair is being cut - both through the razor and via my face's skin. If I've done my normal 3 passes and all feels right, I stop. Very rarely do I search for non-BBS spots.
I still can't get a full BBS (because of my skin, my hair, and the ingrowns/acne it can cause). I usually stop after an XTG pass on the cheeks (if I shave them). On the area beneath the jaw, I stop once I can't really see any hair unless I get really close.