This is a (partial!) checklist that I would use if I were looking for a riding pony for a child. You could probably find most of these in books
Questions to Ask
1. Has he ever coliced? foundered?
2. Has he ever been injured? Sick?
3. Is he up to date on worming? shots? What vaccinations has he had?
4. Have his teeth been floated? when was the last time?
5. Any bad habits? kicking? biting? striking? stall weaving? cribbing?
6. How does he get along with other horses? Where is he in the pecking order?
7. If he is registered, are the papers in the name of the person who is selling you the pony (people have gotten burned big time on this one!) Is he actually registered? Or just registration eligible or registration pending?
Things I would observe
1. What is his overall condition? Weight? Coat? Does he look wormy?
2. Take a look at his teeth. Is his bite even or at least "passable" (what is passable depends on what you are going to use him for)
3. What is the condition of his hooves?
4. How does he act with his current handler? How does he act around you? Does he flinch when you touch him? Is he pushy? Does he respect your space?
5. GENTLY palpate his back where the saddle would sit and around his spine. Any signs of soreness?
6. Does he seem to like (or do you think he can learn to like) you or your child who will be riding him?
Things I would ask to see
1. Have him walked and trotted in front of you and to/from you. Observe any problems with gait and possible lameness. I like to see them free-lunged in a round pen if that is available.
2. If they are advertising him as a horse that is broke for a child to ride, ask if you can see a child ride him (not always posssible in every circumstance!). At least ask to see someone ride him.
3. If he is trained to drive, ask to see him drive. Observe his responsiveness both when ridden and when driven - that will tell you how much training/re-training he will need.
4. IF you believe it is safe, ask if you or your child can ride and/or drive him. Be sure to take proper riding boots and a riding helmet with you!
5. The registration papers. Do they seem to be in order and in the name of the person you are dealing with?
6. Have him measured in front of you if you are concerned about his height.
Before I make a final decision
1. Have a vet to a basic pre-purchase exam. Standard cost here is $95 and it is money well spent. It could save you hundreds or thousands in headaches and future vet costs! Try to be there during the exam.
2. Ask the vet specifically if the horse still has his wolf teeth. If so, and you decide to buy while the vet is still there, have them pulled and have his teeth floated (or better yet, try to work this into the deal!). A lot of "behavior" problems with riding ponies is caused by a bit hitting against wolf or sharp teeth when a child who does not have good hands pulls on the reins. And "behavior" problems can lead to a dangerous situation for the rider.
3. If there will be a sales contract, ask if you can take a copy to review before you actually sign it. Personally, I would never buy a horse on payments without a sales contract. If I'm paying full price for it immediately and they hand me the registration and transfer papers at the time that I pay, I might buy w/out a contract if the pony is not expensive and we didn't have any special agreements.
If any of these items are "negative" that doesn't necessarily mean that you don't buy, but it gives you the information that you need to make a wise decision.