This is not always an easy question to answer.
In general, my approach is this: narrow down the field by research and by test-riding what *is* available. Try to get to a point where you have a good working understanding of (1) what sort of bike you believe will suit your needs and (2) what will fit you.
It's certainly likely that there will be no local shops that will have a sample of the bike you want in your size. Especially touring bikes. So you'll have to work with a local bike shop that will order the bike you've identified, in the size that you need. You will almost certainly have to put down a deposit.
There has to be clear understanding on both sides. They understand and agree that if you don't like the bike, you're not obligated to buy it. You, on the other hand, need to be serious about buying a bike, and ready to complete the deal if there's no problem with the bike. Don't order it just to test ride it, if you really aren't prepared to buy.
Once you've ordered a bike from a shop, it's hard not to carry through with a purchase. It may not be the bike you first ordered, but they will probably want to keep trying to find you a bike you *will* want to buy. You can turn your back on the whole deal _- but it might be hard to keep using that LBS in the future.
Some of the more popular touring bikes -- Cannondales and Treks -- are a bit easier to find if you call around, but not if you're distant from urban centers with lots of big bike shops. Sometimes you can make inferences: ride a Novara Randonee at an REI store, for example, and that will tell you something about how well you might like a Trek 520 or a Bianchi Volpe or a Fuji Touring, all of which are somewhat similar in geometry and frame design. (Emphasis on the "somewhat.")
RichC