You would pull their Account transcript.

Stimulus info can be found at these links:

https://www.irs.gov/payments/view-your-tax-account

https://www.irs.gov/coronavirus/economic-impact-payment-information-center

https://www.irs.gov/coronavirus/get-my-payment

https://turbotax.intuit.com/stimulus-check/

"Is it possible that a client hasn't received the second stimulus, files their taxes and receive a recovery rebate credit, then get the stimulus in a separate payment?"

At this point, you turned the stimulus money into part of their tax return, so the rebate credit creates a net positive or negative tax return amount. Is that what you are asking?