Following one thread to another this morning, I came across the idea that one can place an indicator on a blog or website to show whether or not you're in world. Torley Linden posted to the thread that MeraTalk is looking for testers for her version.

This will apparently be easier to do once 1.10 ... or 1.9.1 ... or whatever number they are giving it now, comes along.  Scripters will understand why. I don't.

Max Case also has one on his site, apparently. The world seems to be divided between those who want to get away from people (and be able to be invisible in-world, even to friends) and those who want everyone to know where they are, all the time.

Max Case has set up a site to enable people to check which other sims share their server. 

Looks pretty useful, especially for those sim owners who suspect that their sim is paired with a club-heavy sim which is taking more than its fair share of resources.  However, the sim pairings have been trawled by people in world, by reading the data in-world. If this is kept up-to-date, that's fine, but sims change servers fairly often. It is certainly better than no information, though, and sim owners can check up the information by cross referencing the sims that the system says are sharing the same server.