find a small cage that you can easily carry around, but will also hold what you want to buy. also take extra cages, you will probably need them!
ask how old the birds you are interested in are, most people will tell you the truth. also look the birds over, if it looks like an old bird it probably is.
as for what to expect, no way of telling. i drive 3-4 hours to check out swaps and auctions, usually i ma dissapointed when i get there. some are big, [with around 50 or more vendors], some are more "merchandise" than poultry. others are so small that every body else selling are kin to each other. [that also comes with certain areas of the country].
you can bet you will see something you havent seen before, and you will see something you cant resist. [i.e. take extra cages!]
dont be suprised if you see people you know, this gets more evident when you get farther into poultry.
be ready to have fun no matter what else, that makes a bad swap better.