They can get a pretty good idea of the rocket origin with the telemetry data available almost instantly from Iron dome. Between that and the likely drone coverage of the area by Israel, the bad guys have to set up hastily, fire, then split before the counter strike arrives. Thankfully, that usually means poor results for the initial launches.
So why are the Shiites in Lebanon (Syria & Iran also) firing on Israel now? The Syrian rebels are Brotherhood Sunnis (Al Qaeda, Egypt, Libya, etc.).