Originally Posted by Demise

For petitions filed on or before January 14, 1998 there is no physical presence requirement.

You do not have to be on the petition, you only need to be a spouse or an unmarried child under 21 at the time it was filed.

You prove it by finding the approval notice of the petition and proving that you're related to whoever it was for.

If petition was approvable when filed but ultimately denied/withdrawn you need to prove it existed and that if things didn't take a sharp turn it would be approved. (E.g. a petition by an uncle withdrawn because your mother died, or petition by a company that went under during pendency).