I'd first do a spook test. How does he/she react around plastic bags, tarps, mail boxes, cars, cats, sudden noises, rattling noises, things popping out of bushes, etc.
Things to look for: Little to no spook. Little to no foot movement when spooked. Curiosity versus fear at new objects. Little to no change in temperament when faced with a new object.
Things to avoid: Skiddish-ness, rearing, bucking, wild eyes, tensing of the body, flat ears, dancing feet, hot temperament/excited after facing new object.
Next would be the temperament test. How does he/she react to standing tied? Are they fidgety? Do they walk quietly and easily? Do they respond well to vocal commands? Do they have a desire to work, or are they lazy? Do they take to learning or are they dumb as a brick?
Things to look for: Doesn't chew/swing around/stomp/paw (flys are OK) when standing tied for longer than 15 minuets. Respond well to vocal commands, or learn vocal commands quickly. Laid back, but ready to work. Intelligent eye, doesn't have a dumb or stupid look.
I'd also work them in halter with lead ropes, stimulating a driving scenario. I wouldn't stand behind the horse or use overly long ropes, but work them with vocal commands and see how they react to having things laying on their back/butt.
I'm looking to maybe drive my gelding Sammy. He's a perfect candidate for it, he has a good working drive and when he is startled (rarely, I danced around him with a plastic bag on a stick yelling my head off and he just ignored me) it's more of a brief tensing of the body before he gives whatever spooked him a curious glance.
I'd say almost any horse without a super high drive can be trained for driving, but if you just want to go on a lazy Sunday trot in your buggy, thats about what I'd be looking for.
Wait! I'll fix it....