I ask everything I could think of, so think of everything you want to do with the pony and get chatting.
Who owns him? How long for? Who rides him? What do they do with him? Has he had any time off due to injury/illness? Is he up to date with vaccinations, teeth and worming?
Is he OK with living out? Is he Ok turned out with other horses or on his own? Does he respect electric fencing? Is he ok to catch? To lead?To groom?
What is he like to hack? Alone? With other horses? In open fields? In traffic? With cows, pigs and sheep? With tractors and trucks? Loose dogs?
Has he competed at all? Do they have results, sheets and photos?
Arrive early, you might find them lunging a lunatic or running round a field endlessly. See what he is like in the stable, touch him all over, check his legs. Ask to see him trot up, many a horse come up lame! The owner must ride first, do not accept any excuses. Beware of owners giving the horse a lead by walking in front of it.
If he seems good and your DD likes him in the school, ask the owner to pop back on and walk around the yard/down the lane. Ask the owner to go out and come back and go out again (this can set off the grumpy ones). Ask for a car to be started and driven near the pony.