I don't know what kind of documents you are asking. For sell by owner. A lawyer should be involved early. you don't need to check anything documents before you decide to buy and agree upon price. After that your and his lawyer would involve to write a contract, then lawyer start to check the documents. Your lawyer would show you what they check and the result, you decide to sign or not at that time. You could ask any questions, but only ask the docuemnts after you intend to buy. Seller don't like show their private documents to anyone, unless it is a real buyer. It is understandable.
Don't forget to buy title insurance, it is only 300 dollars. but save you when there is a trouble.