How To Find And Write Check Routing Number . The third number will be the check number within the book. Knowing how to locate these important numbers is useful for setting up automatic payments for monthly bills and filing forms for actions such as direct deposit. ABA Routing Number Find on Checks & Search Database of Routing from www.aba.com Use the checksum algorithm to find a missing check digit. The answer is evenly divisible by 10, so the routing number is valid. If you have a paper check, you can find the routing number easily.