Welcome to BYC. Glad you decided to join our flock. I'm a chicken person and don't know much about ducks (although I've had a few), but I have to admit that ducklings are the ultimate in cute. You should post this on our duck section at https://www.backyardchickens.com/f/42/ducks and take advantage of our duck experts there. Please feel free to ask any questions you may have. We are here to help in any way we can. Good luck in getting your Pekin ducks.
I would suppose it's a lot like evaluating any poultry, no coughing, no discharge from eyes, nose, ears, mouth. No bald patches, red, inflamed skin. You'd look for a natural stance, not favoring any leg, wings that match on both sides, not one drooping or carried differently. No obvious sores, punctures, . You can also post on the ducks thread on BYC. Know what the breed should look like. Good luck in getting a healthy duck.