Also check the feet. I do a lot of auction watching, and I see a lot of birds come in with those leg mites. Their feet should look nice and kempt, not revolting. When I purchase from auction, I look at EVERY SINGLE BIRD, if one looks like it's not healthy, I toss my buyer's tag in the garbage and scout the flea market for junk instead. And, when visiting another flock, or an auction especially, when you get home, IMMEDIATELY remove your clothes and toss them into the washer, and clorox the crap out of your shoes and wash yourself up. So far, this has worked well for me. We're also fortunate enough that a government group (I forget what group they're with) comes and does testing on the birds at auction. Of course, they do a random sample, not every single bird, so that's only a partial precaution.
I like the feet of a bird not just looking for illness, but it can also help you guess how they were kept if they're auction birds. If their claws are ugly looking, then they haven't been scratching much. I only choose stock that has nicely "manicured" toenails. Of course, my ducks toenails always look like trash, but I do go through the flock once in awhile to clip anything that's out of control (duck toenails, turkey beaks if they're too sharp and long, etc). But, if you see long toenails on a bird, then you know that they haven't been kept up with.
