You need to ensure that your check registrar is up-to-date with all the checks you have written, deposits you have made and debits. You can do this daily, as you write a check, or on a weekly or monthly basis.
Sit down with your cancelled checks, your ATM receipts and your receipts for items you purchased using your debit card. Make sure each item you bought and each check you wrote is listed on your check register. Make sure each deposit is also properly recorded, including direct deposits.
You can compare this list to your online banking transaction list and/or to your monthly statement in order to ensure that they match. However, do not assume that online banking or bank statements are always correct. It is important to check each item one by one in order to determine that the proper amounts were debited or credited.