Obviously the first way is to be in a good, active, busy guild, but alas, that is not an option for me right now.
My current practice - if I see a group that's got an LFM for a cleric, I'll put myself on 'LFG'. If they notice and ask me to join, that means they're paying attention, and probably know the game reasonably well.
And if not, well, then I probably didn't want to group with them anyways.