Sorry, but I have to speak out. NO ONE comes to my property to see my birds, and that is a silly thing to evaluate a breeder by. If anything that would be a big strike in my book. To those who are disappointed when I won't let them come visit, oh well. My family and flock safety come first, and I'm not the local petting zoo.
Disease - (very easy to transmit). VERY!
Theft - yes, there are bad people out there who will come back and take what they want. Remember those birds you just pointed out and bragged about. So does the visitor.
Ask questions. The right ones will let you know the important stuff. Do they belong to the breed club, where did they obtain their birds, do they exhibit (NOT at fair level)? Obviously, don't buy birds sight unseen, unless it is someone with impeccable references. Take the time to go to an APA/ABA show. You will learn more in one day at a show, talking to breeders, than one year on BYC.