It would be helpful if people were clearer about what is open and closed for new players. I think there are a few games that "look" closed but are still sort of open for one extra player, but it's a bit awkward to barge in and ask :/
(why yes I am looking)
I always endeavor to make an explicit post announcing that "Recruitment has ended," or, "Recruitment will end on X day." Most others also do something like this, at least most others who run games in which I am interested. It may be more problematic if recriutment is re-opened for a replacement player or something, but most gamemasters would not be offended by an inquiry by an interested player in the OOC thread for a game, and you might even secure an invitation to a game that was not actively looking for new recruits, because sometimes a game that has been going on for a long time can gain new vitality by the introduction of an enthusiastic new player.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.