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.