Glee - trivial question

[dalek]

When did Mercedes and the others find out that Finn is not the father of Quinn's baby - don't remember that at all but suddenly everyone seems to know

ages ago. i think puck told mercedes

I think I must have missed an episode then as I have no recollection of that. Oh well now have a reason to rewatch the whole series on DVD

They found out the episode before the big comp I think and then Finn found out, cue him not going to the comp but turning up at the last minute and then they won (despite another group stealing their song list and so they had to change everything at the last minute).

Yep Puck told Mercedes and of course she told everyone else.
Rachel guessed and then told Finn.