I try to be "customer centric" when making those decisions. Most people will ask "what do I owe you?". That makes it easy.
Figuring that out on the phone is the best bet. I will say "Sometimes there is a fine line between a diagnostic/consultation visit and an estimate. If this an estimate, there is no charge, but if it is more of a consultation, I do need to charge....the kids get fussy if I don't feed them."
It does stink when it takes a full hour to not get anything out of it...but that doesn't happen often. I've also gotten calls from that guy's nephew, or daughter, or neighbor a year or two later so leaving somebody with a good feeling about your visit isn't bad.