1. Visit midday. Take notice if the horses have any hay left to eat and if the water buckets are sufficiently full. Most barn owners tell me on the phone that their horses get plenty of hay which is usually BS. If you don't see any hay on the ground, ask the BO why they have no food. The answer will determine whether you should just walk away right then and there. Also if the water buckets are empty, just leave. It won't change.
2. Look at the horses. Are they outside or in their stalls? Do they have shelter from passing storms? Are they ribby or fat and happy? Are they happily dozing or do they look stressed out?
3. Fencing - Simple stuff. Is the fencing strong and sturdy or about to flop over? Any jagged pieces sticking out? Any nails? Pieces of wire?
4. Ask about the feeding program. Are all horses fed the same quantity of grain regardless of their needs, or is it talored to the individual horse? If the answer is all get the same and you're welcome to buy your own weight gain supplement, run for the door. I've been at barns where I thought I was saving $100 a month, but paying it all back out in supplements to keep weight on my horse.
5. Check the footing in the ring or rings. Too deep - leg injuries. Too hard - same thing, just different injuries.
6. Ask other boarders if they are happy with the place when you can get away from the BO. If you have time to make your choice, visit a few different days and times.