A short monthly review can help you find unfamiliar payments, unexpected charges and changes in recurring bills.
First, match major credits and payments against your records. Second, group recurring charges and note changes in amount. Third, list anything you cannot explain. Merchant descriptions can differ from familiar brand names, so check a receipt before deciding what the payment was.
If a transaction still looks wrong, contact the institution through its official channel promptly. Keep the date and reference number for follow-up. Save any records where you can retrieve them without sharing account details unnecessarily.
Further reading: CFPB spending tracker.