Good question, and not easily answered. But I'll try.
Generally, each roleplay will have at least 2 threads: an "in-character" (IC) thread, where the actual collaborative writing takes place, and an "out-of-character" (OOC) thread where meta-discussion takes place about the roleplay. Another way to consider these is that the OOC thread is inhabited by the players (or the authors, if you like), and the IC thread is inhabited by the players' characters.
I'm sure this makes some level of intuitive sense, so I won't belabor it. However, I did bring it up for a reason: if you are unsure whether a roleplay is accepting new players, read the first post (or posts, in the event the OOC thread has multiple posts detailing the rules of the roleplay) of the OOC thread. They may say in that post whether they are taking on new members. If the posts don't specify, then ask the current players (and/or game master (GM)) in either the OOC thread or in private message.
Keep in mind that the above is general advice. There are some things you can look for to make the dilemma easier.
Tags are used on SC for each thread. You can opt not to use them, but from a player's standpoint, they can make things easier. You can expect a thread with the "Open" tag to be accepting players, and you can expect one with the "Closed" tag to not accept new members. The absence of a "Closed" tag does not indicate openness nor the complement. But it's a good place to start.
Another good place to start would be the bulletin board. If you are at all familiar with MMO culture, this is sort of the "looking for group/looking for player" area. If you're not, then suffice it to say that this is an area where people pitch prospective roleplay ideas and see which ones gain enough interest to start.
The bulletin board is helpful in that you might be able to get into another roleplay on the ground floor. If you're a founding member, you will have secured your spot in the roleplay, right? Another thing is, humans being the fallible critters they are, there may be details in even a started RP's bulletin board post that are for some reason assumed in or omitted from the related OOC post which may shed more light on it.
But if all else fails, ask. That's the general advice. Be respectful, of course, and communicate well. Good communication is the single best defense against conflict.