It sounds really simple: get on a bike, ride it around the neighborhood, head back to the LBS, and do it again on the next bike.
What do you do if you're looking primarily for a comfortable ( or any other attribute that's important to you ) bike, and you'd like to compare two, at different shops five or ten miles away? The road conditions are pretty different, and I think if I try to concentrate on how much nicer each one is than my bike, I'll get myself into trouble. I can't ride one to the other's shop and do an A/B comparison, because I have to leave my stuff to get the first bike.
I assume everybody does a shortish loop around the neighborhood the LBS is in, looking for how much effort it takes to accelerate, to keep the bike up to speed, how it climbs and descends, etc. Do you bring a GPS to compare speeds? Anything else that isn't intuitive, but that you find helpful?
One local shop, probably the closest one to my apartment, told me I can take one of their bikes for a half-century test ride, but that it'll have to be during a week day. The anti-commute! I like the bike a lot, and the work they've done in the past has been top rate, so I'd like to check out my other options before I take them up on this and fall in love with N+1.