OK, purchasing from an "unknown person" should put you on high alert status straight away.
Basically, there is no way to be certain an iPad is not stolen. It is incumbent upon the seller to prove that it is his to sell. Various documentation may be acceptable, such as an original invoice, AppleCare registration, original box, etc. The original wall charger should be included.
If you have the slightest suspicion it is stolen, walk away. Possesion of stolen goods may be a crime where you live. If you are satisfied the seller is legitimate, make certain the seller provides a bill of sale, indicating transfer of ownership to you.