My advice is when at all possible, choose a rediculously rainy day after it's been raining for a couple days. Nothing tells you more about a barn faster than their mud plan.
Are the horses standing in the rain or are there shelters?
Have blankets been put on? Is this even an option?
Do they turn out when it rains or are they stall bound?
When they are turned out in or after foul conditions, is there any mud? If so how deep is it?
How is the barn cared for in bad weather?
I know it seems like something stupid to pay attention to but making pastures and turnouts mud-free cost A LOT and they're not something you can identify unless you're in the right weather conditions. They're also something that everyone SAYS they have but very few people do.
Horses NOT standing in mud to me says, these people put their money where their mouths are and really do care about their horses.
Silly, but easy.