[#3Fan] Congress decides what constitutes proof of an act.
They did precisely that. The proof consists of affixing the official state seal.
http://press-pubs.uchicago.edu/founders/documents/a4_1s6.html Article 4, Section 1
[Volume 4, Page 472]
Document 6
An Act to Prescribe the Mode in Which the Public Acts, Records and Judicial Proceedings in Each State, Shall Be Authenticated So As to Take Effect in Every Other State
1 Stat. 122 1790
Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That the acts of the legislatures of the several states shall be authenticated by having the seal of their respective states affixed thereto: That the records and judicial proceedings of the courts of any state, shall be proved or admitted in any other court within the United States, by the attestation of the clerk, and the seal of the court annexed, if there be a seal, together with a certificate of the judge, chief justice, or presiding magistrate, as the case may be, that the said attestation is in due form. And the said records and judicial proceedings authenticated as aforesaid, shall have such faith and credit given to them in every court within the United States, as they have by law or usage in the courts of the state from whence the said records are or shall be taken.
Approved, May 26, 1790.
See post #767.