With experience, you generally get an inkling of how much a customer has to spend within 5 minutes of walking through the door, but never judge people as I've been in brand new houses with a new merc in the driveway and they're so hawked up in payments, that they have little spare cash left to spend. Talk to people and all is revealed. If someone says it's a quick easy job, and are not too bothered with what they want, then they want it cheap, if they ask for options, or to select from a catalogue then they want a decent job with money spent on it. The best thing is to always be friendly, but pop questions into the conversation without probing when doing so.