> how do they check whether the contestant is conscious?
already answered but they'll apply pressure on your hand (or similar) and you need to apply pressure back
> How does the contestant know what his limits are before passing out?
When you hold breath for a long time your body will have muscle contractions. The time that needs to pass for each contraction to happen varies from person to person but it is quite consistent for each person. So free divers can know that they are good up to X contractions which will take after X minutes in certain conditions. The fun part is you can train to experience your first contraction by holding your breath while laying down in bed.
I think this is a self selection bias.
In a group sport like club cycling, it can be everyone's responsibility to make sure that your fellow riders haven't gone either hypoglycemic or into heat stroke. We all watch each other so we can go a bit harder and the people who can still talk keep tabs on everyone else.
I understand that with submersibles and astronauts there's a bit of this going on as well. Everyone is watching everyone else for nitrogen narcosis or hypoxia. Maybe another reason the Navy doesn't like assholes on submarines. How can I tell if you're being a jerk today or we need to check the CO2 sensors? Better to notice Lieutenant Ivers only gets short with people when his blood ox goes a little south.
If you go to solo walking or running, now you are the only one tracking your mental state. Now you have to use your own judgement to try to detect when your judgement is going away. It's... tough. Personally I think it's easier if you've already had practice on team settings. But it's still tough.
Same thing with alcohol. There's a reason bartenders don't serve drunks. No judgement anymore. You should have put the glass down half a drink ago and had some water instead. And I think you can only learn that safely by slowly sidling up to it from the safe side, and have someone to look after you if you go a little fuzzy.