Do you know as soon as you are riding on a flat?
My question is, if you do not look at the tyre and you get a flat, do you realise that you have a flat? 


Typically I can catch well before the rim starts bottoming out without looking. If the bike ever feels "off" I will give a quick check by it plopping my self down on the saddle.  If the tire is going down this will usually bottom out the rim which I know shouldn't be happening. At this point I start visually checking things.  If the bike feels a bit soft (I.e., we have lost some pressure but not enough to bottom out the rim) I will give it a quick visual by looking at the tire bulge when I bounce on the saddle.  Finally if the bike feels solid I know it's my imagination and continue riding.


  Or you may end up driving for some kilometres before you take notice? Especially if you are tired after a long ride?


After a while these checks become second nature and you start doing them without thinking even when tired.