The airline can also play a part in identification - to look at the example you mentioned the 737-600 or A320. if you think it might be one of those then unless it is SAS or Tunisair (who operate both - not sure if any other european operator do) then it is likely to be an A320.
SAS also operate the 737-800 but the 2 over wing exits will help there.
The really nasty ones are those who operate a lot of similar types ie Air France who use A318, A319, A320 & A321 to LHR.
Winglets can also be used but again in conjuction with other factors namely who operates aircraft (long or short haul) with winglets. Emirates are easy as it will only be a 777 (or 767 for AA/UA) for non winglet as will AA or UND (no A330 / A340 operated)