If possible always take a fixed reference point from which you know it is supposed to be level or straight up, like a light mast or something.
I thought we had all decided in here quite some time back that light poles are not a good source of level/vertical?
I wish there were a good building vertical that he could use in that photo as most of those radio towers aren't exactly level either.
I'll go with wheel axles on this one as they should be right across from each other. On that note, I guess you could also use the spinners in the engines...
Canon EOS 7D & 40D -- 100-400mm L IS -- 70-200mm f/2.8L IS II -- 28-135mm IS -- 10-22mm -- 18-55mm EF-S