A few other things I ask:
Is the house/flat owned or rented, if rented are they allowed pets and I would also ask for landlords name, address and phone number to check that they were allowed pets.
How long will the cat be left alone for each day?
Does anyone in the house have allergies to pets, asthma etc?
If they were placed in a position where they couldn't keep the cat what would they do?
Have they ever had to give up an animal previously, if yes why?
If they haven't got any pets at the moment ask about their previous pets, you can judge a lot by how people speak about their pets.
What do they expect from a cat?
How do they plan to train/discipline the cat?
If they currently have pets would they be happy for you to contact their vet for a reference? If they say no alarm bells should ring!