You make the person sit in front of a notary, he confirms their identity and reads them the document they are about to sign (to make sure they know what they si

You make the person sit in front of a notary, he confirms their identity and reads them the document they are about to sign (to make sure they know what they sign) and then confirms that they personally and voluntarily signed. Of course, you can only check for immediate coercion. You can make sure nobody has a gun to their head, but you can hardly check that nobody has a gun to their children in the car outside, or their brother that lives on a different continent.