Matching payment method/date/cardholder name to bank statements. You can prove that you paid X amount of money to Y company on Z date, and the matching itemized receipt received from that company indicating that you bought A, B, and C products that may have been tariffed.
Harder to prove if you paid with cash or gift card. Doable, but probably more trouble than it's worth to effectively collect pocket change after lawyer fees take their cut from the class action settlement.