A lot of them will indicate if they are still Open or Closed.  It never hurts to ask in the OOC Thread or Interest Thread or even send the GM a Direct Message.  Even if the answer is "No, we're closed now," they might be able to direct you to another RP they know is open or they may be willing to put you on their short list for folks they will message when they start another RP.

I know I've made notes to message certain people when I get such and such an RP started and I've gotten messages from previous writing partners when they start a new RP or join an RP they think I might like.