I think i've only been in similar situation twice in bad haze, in both occasions I asked the AC on final to report their altitude, in both cases they were at some 300-400 ft and descending. For me that tells me where to look because you roughly know where you need to be at 300 ft from runway to maintain a half decent approach angle and that helped me spot them and decide on my next steps. If you cant see them and they're not talking to you, I would only extend if there's no-one else in the circuit or if the other party is also happy to extend. Communication is the key, I think.