as roflmao456 said, it is possible but its highly unreliable.
The first place I ever saw this was years ago in spng where they had convo enter/leaving notifications. But later, these couldn't be distinguished from requests to the server to get contact info (for the small window that opens up showing your myspace blog thing).
Since this ambiguity couldn't be removed, the feature was scrapped (false positives ftl). This is what happened iirc and nothing has changed since then
/ 
