The Prepurchase Exam
Whether you want the horse checked by a vet before you finalize the sale is up to you — and it will be at your expense. Prepurchase exams are money well spent, especially if you're planning to use the horse for a particular sport or discipline. It's sort of like a home inspection before buying a house. All large-animal veterinary practices have a standard exam they administer. If the seller won't allow a prepurchase exam (also known as a vet check), suspect something fishy and walk away. Any reputable seller will expect you to do a vet check. Also, choose the veterinarian to do the exam yourself; do not find a vet based on the seller's recommendation. If possible, arrange to be present during the exam.
In a prepurchase exam, the vet checks the horse's temperature, heart rate, and other vital signs, and also performs some general lameness and range-of-motion tests. You may request other tests as well. For instance, if you are buying the horse with a specific performance activity in mind, such as jumping or dressage, you may want the vet to X-ray feet and leg joints. If you plan to use a mare for breeding, ask the vet to check the horse's reproductive system.
There are no perfect horses and no perfect scores. What you are asking the vet to determine is whether the horse is healthy, sound, and fit enough to do what you plan to use it for. The vet will share the results of the exam with you and the seller and provide you with a written copy. If the vet finds something out of the ordinary, you must determine whether it is something you can cope with or whether it will prevent you from doing what you want to do with the horse.