You are right paint can be a misleading indicator of aircraft condition , although in my experience, more often than not the airplanes that looked like a piece of junk,were junk
. I use two indicators for making snap judgements on aircraft. The condition of the windows and the condition of the aircraft belly. If the windows are not hazy,cracked, or crazed, and the belly is clean than even if the airplane looks a bit sad, it is probably well looked after
I remember reading in the Jeremy Pratt books that you should check the fire extinguisher & medical box. If they are there and serviced then the aircraft is probably well looked after. From my brief experience of flying a limited set of different aircraft it seems to hold somewhat true.