How can I authenticate payment processing requests as a Shopify partner?

[Thuanmd]

I'm a Shopify partner. I'm implementing "Processing a payment": https://shopify.dev/apps/payments/processing-a-payment.

Step 2 in document: "Shopify sends a backend request to the payments app, specifying the amount and currency to be charged."

It does not have any signature to authenticate. How do I know the exact request from this shop? 
Thanks.