Itching and bald spots on dog

Question
We are looking to adopt another Wire Fox Terrier in addition to the one we have now.  One turned up at the local Humane Society.  We went to look at him today.  The dog is 4 years old and seems alert and playful, but he is missing hair next to his tail on both sides and down his rear and back of the back legs.  The adoption sheet said he has food allergies and he was being fed a lamb and rice diet.  According to his data sheet, he has never been to a vet.  The policy at the Humane Society is that you can return the dog for a refund if a vet check turns up something serious, but I'm afraid to even take him home for fear he's got something that our other dog could catch.  My wife thinks maybe there is an anal gland problem in addition to allergies because of his scratching and where the hair is missing.  I know it's impossible to diagnose a dog you've never seen, but what insight can you give about food allergies and/or anal gland problems?  Any input appreciated!  

Answer
normall if a dog has a food allergy problem, the problem would be at his face too.
anal glands do cause itchiness and scooting.

not only do you have to worry about anal glands and or food allergies, you have to think if this dog has MANGE.

for an adoption sheet to say he has those diagnosises without seeing a vet is illegal in many states. since a vet did not diagnose the problem, the shelter is playing dr.

if you want him, take him to the vet right after the adoption and see whats up then make the decision to take him home.

good luck!