Hi MPW, welcome to the forum
Interesting you should ask. Others have been requesting that Discourse have a way to show online status.
Currently the closest thing to online status is “last post” and “last seen”
For example, if I click on the avatar or your name in your post I see
“Last post 7 mins ago”
Are you still online? Most likely, but maybe not.
If I go to your Profile
I also see
“Seen 7 mins”
Other than that, there is no way to know for certain whether or not any given member is online