First step is to ask for references

A reputable breeder won't hesitate to give you references. Also, check for the cleanliness of the aviary (if it is a closed aviary, don't panic, as this isn't unusual) or the area where the babies are kept.
As what type of diet the babies are on. If babies are currently clipped, ask the breeder how long they were able to fly before s/he clipped them. Ask if the babies have been taken into the vet (again, don't panic if she says no, as most breeders don't do this). Ask if there is a guarantee on the bird (if there isn't, RUN!! LOL). If there is a guarantee, ask how long it is and what it covers.
Don't feel as if you are asking too many questions of the breeder. It is your right and if s/he seems to be getting perturb by the amount of questions, then you might want to pass. Also, ask if you can contact the breeder at any time if you have any questions or concerns.