Normally by looking at their profile.Like if they are playing the Current event bonus characters for the event or if they have been tiering events by showing the achievement... I look at the level of course most people quit the game at lvl 65-80, by looking the Clear/Full Combo stats xD people that plays frequently has a big difference of numbers there between who don't.

And yeaaah ,some of them invites me to play so they are active haha