"Certify" does not mean the same as prove with evidence. I don't think there's a specific threshold that has to be met there other than the parties agreeing it has been certified. It can even come down to as much as "trust me bro" from the president.

The definition of certify is literally "attest or confirm in a formal statement"

Besides, there's no process through which Congress would question or investigate whether the president really can or cannot certify whatever he claims about this matter.