Chickenlover 13 and Drumstick Diva gave you some great advice as to what to look for with the chickens themselves. But I understand if you are not familiar with a good chicken habitat then here are some things to look for:
If there is no run and he doesn't let them range (wander around outside) the chicken coop needs to be big enough so each chicken can have room to exercise and attend to their own hygiene. How big is the shed? Is it crowded? There is a standard formula for determining the size of a coop, but since you are not out there with a measuring tape it is probably ok to just estimate by how much space it looks like. Do the chickens have empty floor space available?
Chickens like to roost. This means a higher area for them to perch on for sleeping and also for them to just get above the floor for a while. Look for 2x4 s or wooden branches set horizontally at least a foot above the floor. If they do not have this they are prone to sitting in a corner which doesn't let them air out so to speak.
There should be some kind of litter on the floor. Litter: Straw, grass, or wood shavings are used to absorb moisture from droppings. Not kitty litter but it may be sand. Dry poop is not as bad a wet poop. Chickens also need a place to take a dust bath. This is how they clean themselves under their feathers and prevents mites.
Fresh water/food. Water is harder to keep fresh than you might think, if you set out a dish a chicken will hop in it and get it dirty very quickly. I use a bucket with drippers that the chickens interact with to get water, I also have hanging feeders that the chickens cannot climb into and scatter the food around. What you can look for is hanging buckets or pipes with drippers/nipples.
Nesting boxes: Chickens like a private little place to lay their eggs, if they don't have these they will lay their eggs almost anywhere and the eggs can get dirty, cracked, or just overlooked until they have gone bad.
I would suggest you use your instincts and if something says bad or dirty to you or if the owner seems shifty I would say avoid it. But if everything seems fine, it probably is fine.