First and foremost - get a vet check. The vet will rule out any issues (flex tests, etc.). If you get the book, Draft Horse's an Owners Manual by Dr. Beth Valentine, that is our BIBLE! Will give you all the info you wanted on draft horse health and care.
This link gives you what you should find in a good Perch: http://www.percheron.ca/characteristics.htm
As for the rest:
Ask the owner to ride the horse first, then you ride. If the owner will not ride first or give excuses, be suspect about it.
Take a good hard look at the feet and ask about the farrier work and how the horse stands for the farrier. Have the owner pick up all four feet.
Ask if tack comes with the horse.
Ask about the diet, have they had it on a high fat diet? NO grain for drafts!!! If they haven't it's not that hard to switch them over.
Ask how the horse is with fencing. Take a good look at the where the horse is kept, the stall, the turn out. What other horses the Perch has been turned out with. That will tell you a lot.
Print out this horse buying checklist. It's a .pdf file. It is the BEST checklist I have ever seen. 8 pages of in depth questions to ask. http://www.equinelegalsolutions.com/assets/HBC.pdf
Good luck! Having a Percheron will be the best thing you ever did!!!!