If a player is sitting out, most of the time there is a banner that says "Sitting Out" at their position. However, often there appear to be players that are not at the table, as their hands and blinds insta-fold, but it does not say "Sitting Out."
In these cases, they are obviously not at the table any longer, as their hands repeatedly fold instantly. Several times I have seen players blind out of a game when they aren't there, but it does not say "Sitting out".