[quote=@Animus]
IMO one of the biggest signs is when the latest posts in the OOC are all about them saying they'll post tonight, the day after and etc.
[/quote]

Quoted for truth, my friend.

Another is when you've got an existing roleplay with someone, and they ask "So, what do you want to do?". Tell tale signs they have no desire to continue. Best to just part ways right that instant, rather than try to piece it back together because it'll wind up dying regardless.