1. Bank and card accounts
Every account the business uses should agree with its statement at the last day of the month: current accounts, savings, credit cards, Revolut or other e-money accounts, and payment platforms. If the balance in the books differs from the statement, find the difference before going further, because every other check depends on the bank being right.
Look for transactions sitting in a suspense or uncategorised account. Each one is a question someone will ask later, and it is cheaper to answer now.