I only know if they say. It's common for people who post like that to say "this is done, I'm just posting it one chapter a day/couple of days/week" so people who worry that a WIP will be abandoned don't need to worry and can read. Also possibly to garner readers like me, who don't read incomplete WIPs.

There are other ways to tell, and you've nailed one of them there. If they're posting on a very strict schedule, that's sometimes a hint. Other times, the chapters are already outlined - WIPs usually are 1/?, 2/? etc. If it's 1/13, 2/13, etc., I think most people presume the story is already written and just being posted serially for whatever reason.