Import the GSTR-2B the portal issues for a period and reconcile it line by line against the vendor bills already in your books. What matches, what you recorded and they did not, and what they filed and you never booked.
The reconciliation that decides your input credit. Claiming against a bill your supplier never filed is the mismatch that comes back months later.
Three outcomes, and the two that are not "matched" are the ones worth your morning.
The reconciliation runs against the bills already in your ledger, so there is nothing to prepare before you can run it.