How to check if user is online

[portes]

Hello,

I want to know if it's easy to do an IF that checks if the user is online and then do something. I don't want it to do something to that user, I want that it shows that to everyone.

For example, a different postbit background if the user is online in the moment.

I've seen some addons that make similar things, but if there is a cleaner way to do it, I will be very grateful.

Thanks a lot.