"But if they claim less, I want proof"

They can claim less because they didn't qualify for the full amount (phase out). I think you are referring to those who expect you to file for the difference, right? Because their Account Transcript should show why they didn't get the full amount otherwise, even though a larger amount got credited, if they had, for instance, child support owed.

I earlier posted this link to an account transcript image:

https://www.irsmind.com/irs-notices/irs-transcripts-now-provide-stimulus-payment-information/