I'll give it a try.
1st of all, see it walking, trotting and galoping (are the terms correct??), =) see it moving in all possible ways, and angles. check for limping.
Look closely and touch the joints, check for heat, swelling or pain of any kind.
Touch the tendons, seek for pain or tenderness.
Check his teeth to see if the age is accurate (keep in mind that it's not 100% accurate, but it will give you an idea of how old the horse is)
See how comfortable he is being touched anywhere, including his back legs.
Check the hoofs, smell them, anything smelling rotten is bad.
Press his muscles from the withers twards the hip, check for pain anywhere.
Check his coat, does the fur look ok? is it shiny? any bald spots?
It will be a good Idea to have a vet with you, or at least someone who knows these things, It's very easy to get fooled if you never bought a horse before.
, seemed really nice.
Good luck ;)