GST return scrutiny guidelines: reconcile GSTR 1 with records, verify ITC differences, and require supplier certifications where large mismatches exist. Officers must obtain transaction-wise outward supply details and reconcile GSTR-1 with underlying records to identify category mismatches (B2B, B2C, exports, advances). For ITC discrepancies between GSTR-3B and GSTR-2A, require supplier CA certification when per-supplier difference is Rs. 2.5 lakh or more and ledger confirmations for lesser amounts; verify supplier tax payment and correct reporting of forward versus reverse charge. The proviso to Section 16(4) applies only to claims made in the extended period; reversals may be verified from the specified return's table or DRC-03.