Going to one's profile, the only indicator saying if the user is online is a tiny green dot to the bottom left of their avatar. Users can choose to remove that and mark themselves as invisible on their profiles.
My suggestion is to make it more obvious that a user is online if they don't have "Invisible?" checked under preferences that is. Our profiles show when we were last active, but it doesn't say, or at least I haven't found any other indicator saying if the user is online or not besides the green dot. Maybe at the top of our profiles or near our avatars it actually says "Online" if the user is online and not invisible. In forum threads, next to our avatars maybe it could be under our blurbs or what not where it says our Title and Post Count we could have it state if the user is online or not.