Once you've joined an association the bonuses will give you an idea of wether a player is active or not. 40% every day = active. 0% = nobody at the helm.
Also through the chat function you can see when an association member is logged on by the blue exclamation mark. Tap that and all members appear bottom right of the screen with logged on member highlighted. Scrolling through past chat is also an indicator of how active the association as a whole is.