go in and look smart, maybe a uniform?
and get some good buisness cards, and just ask, its not hard, "i work locally and was wonder do you have a window cleaner?"
if they answer yes, say, fair enough ... or be cheecky and say ... "would you like an alternative quote?"
if they have a good window cleaner then they will say no. if theyre not, bonus.
try to speak to someone in charge to, not teh counter teenager who doesnt know anything