What are your goals for having chickens? Do you want them for eggs, meat, eye candy, to show, bug patrol, breeding and hatching chicks, sell hatching eggs, a combination or something else? If you want them for eggs and the hens lay three a week, not real good. But if you want them to show or for eye candy they may be just what you want. Determine to yourself why you want chickens and chat with them about how good a fit those are.
Ask if they are vaccinated and if they are, what for. If they are vaccinated do some research to see what those vaccinations do.
Ask how they are being fed, specifically percent protein. If chickens get used to a relatively high protein feed I think they need a higher protein feed than those that are not used to it.
Ask if it is a closed flock and when the last outside exposure was. A closed flock is where they are not exposed to outside chickens that could introduce diseases or parasites. I keep a pretty closed flock. I do not show my chickens. I hatch my own eggs, hatch someone else's eggs, or get baby chicks from a hatchery. I do not bring in any other chickens that could be carrying something. If they are not closed it's not necessarily a disqualification for you. It's more to know you need to be on alert a little more.
Ask if they know if their flock has any flock immunities. It's not that unusual for a flock to be infested with something that could infect other chickens but that flock has developed immunity to that specific disease or parasite. Coccidiosis is a perfect example. It is caused by a parasite that, if the numbers are kept under control, the chick or chicken can develop an immunity within two or three weeks. They will be carriers but won't be affected. I would not consider all these a disqualification, some like Coccidiosis can be managed, especially if you know it is there. But some, like Marek's would be a disqualification.
Like Mrs K I think if they look healthy they probably are. If any look hunched up, fluffed up, and standing around being lethargic, run away. If they look healthy and active they probably are.