How do we know that a person we are following is still active?

True, you can check their most recent activity and look at the date that activity happened. That lets you know the last time they were doing something on myLot. (Except if they were using private messages with someone. We can't check that.)

You can check their most recent activity.

The only way is to see their latest activikty Some Sites show when they last visited the Site

This can be good to know. I just find that interacting with recent posters is the way to go

That's how I check. I just go to their profile and see when they made their last comment or post.

In case they are following you or you have been following them, you could check their activities . In case they have not been active for long , you could drop a direct message to them and see if they respond. If you are lucky , you could find some of them active and perhaps you could be driving force for some of them to return to mylot. In last 8 months I have been here , less than a dozen people returned .Unfortunately not all have been active. Perhaps they were detered by the less number of users or seeing their old folks not active anymore. Add to that the earning potential is less compared to earlier reported times.