Share registers and purchase documents on both sides, showing that the same individuals own both companies in the same proportions. That is the affiliate test, and a mismatch in percentages between the two companies is the usual reason it fails.
Match the percentages before the closing date, not after
An affiliate is one of two companies owned and controlled by the same parent or individual, or by the same group of individuals each owning and controlling approximately the same share of each entity. If Brennan and Tara each hold fifty percent of the Canadian corporation, the U.S. purchase should give each of them fifty percent as well.
Evidence includes the Canadian minute book and share register, the U.S. stock purchase agreement, the U.S. stock ledger, and an organizational chart signed by an officer.
If instead the Canadian corporation buys the U.S. outfit, the relationship is parent and subsidiary and the evidence is the Canadian corporation's ownership of the U.S. shares.
Either way, bring the documents that show control, not just ownership, including any shareholder agreement. A purchase financed by a U.S. lender with a pledge of shares should be reviewed, because control that sits with a lender complicates the picture.
Settle this with counsel and the accountant before the closing is signed.