Know how you can tell when a customer is just there to check sizes and not buying a bike from you? Their VERY FIRST QUESTION out of their mouth is, "What size do you think I am?" There's a pretty fun way to deal with this and I'm still trying to perfect my strategy, but either way, the customer walks out thinking "I still have no idea what size I am. But he sure talked my ear off about bikes!"

"What size bike do you think I am?"
I instantly change the subject, back up the conversation and ask them what kind of riding their doing and that we'll get to sizing very LAST. Poke questions at them, they generally just want a number... and a number they will not get. Sometimes telling them that the correct size is what my job is all about not only gets the customer's confidence in you, but leaves their size still a mystery.

A bike shop needs to be creative with how they deal with the "Guy that steals knowledge and buys on the internet" guy. They know who you are.