Depends on the bike shop. Virtually everybody in the business has had somebody ride off with a bike or two so many shops are reluctant to let the bike out of their sight.
The real question is: "If you ride it and you like it are you ready to buy it?" If your answer is "Yes" try this: Work everything else out first. Negotiate the price and get fitted to the bike. Now tell the sales person to write up the sale with the express understanding that if you don't like it after an extended test ride that you have the right to a no quibble return. Given the opportunity to put money in the drawer and the fact that most shops will have some kind of return policy anyway, I think that most bike shop guys will agree to that offer.