keystring An interesting exercise (which I've done and still do) is to become a foot watcher of good pianists when they play. It's like the feet have a brain just like a pianist's hands seem to have a brain. That may be a good way to answer this question.
I have to respectfully disagree with this. Every good pianist will tell you to pedal with your ears not your feet.
At the beginning you should learn the basic pedal technique like WieWaldi describes above but for more advanced usages you just have to experiment and find what sounds good. I do a lot of different things with the pedals - partial pedal to various degrees, partial release, flutter pedal, sometimes engaging una corda to various degrees - but if you asked me exactly when I do it I'd have no clue. It depends on how much resonance has built up from what I play before, the tempo, the mood I'm trying to bring out, etc. Even the accoustics of the room make a difference and I would pedal something slightly differently in my teacher's studio.
So, IMO watching someone else's feet is not a good idea and you will not be able to replicate what they are doing from their feet alone.