In a conference, when will the decision of best paper (or other award) be made? I am just curious, do they make the decision before the conference?

Or do they watch the presentation of potential candidates first and then make the decision?
It depends.

Some conferences, such as ACM CHI, announce them before the conference even starts. This has the advantage/disadvantage of letting people know which talks they should see. And in my experience, these sessions are packed!

Other conferences announce them at the end of the conference. This has the advantage of keeping everyone at the conference until the very end (people might try to leave after their own presentation...) so they don't miss their own award!