When going to look at a horse keep your goals in mind. Don't be intimidated into buying the horse or pony take your time as it will pay off in the long run.
You will want to observe the horse in the stall or pasture. If in the pasture how do they act around other horses? If in a stall do they turn their hind end towards the owner or pin their ears when they enter the stall?
Observe the owner catch the horse. Does the horse come right up to them willingly or do they walk away?
How does the horse act when tied up do they paw or dance around?
What is their behavior like when being saddled, bridled or harnessed for driving?
Have the owner pick up all four feet and observe this carefully as you will want to look for any signs of the horse throwing their head in the air, snarling their nose, pinning their ears or possibly trying to bite.
Ask the owner to touch the horse all over and observe the horses reaction. You want to know this because there will come a time that you may have to clean the horses sheath or udder. If they don't like their ears touched they may be difficult to bridle.
Look for white marks on the back as this could be a sign that their back is sore because of an ill fitting saddle.
Is the horse spooky? How does it tolerate tarps, slickers or plastic bags?
Do NOT ride or drive the horse first as the owner will need to do this. Make sure they take the horse through it paces meaning have them work the horse at all three gaits (walk, trot and canter). Be aware that most driving horses do not canter under harness unless they compete in CDE's (Combined Driving Event).
After the owner has ridden or driven, the horse or pony if you feel confident go ahead and give the horse a try, but do so in an enclosed area as the horse does not know you. If you are new to driving make sure you have the owner or equine professional in the cart with you. If you are not comfortable now would be the time to ask your friend or equine professional to ride or drive the horse. If this is a pony for a child make sure that a child rides the pony and watch how it behaves.
Take your time and go back several times if need be. There are a lot of good horses out there and you would be better off taking your time then getting a horse with problems. If you are uncomfortable with the horse walk away don't let the owner talk you into buying a problem horse.