Look at the checks associated with your direct deposit account. Some banks list their direct deposit tracking number along with their routing number on the checks they issue. Usually it is labeled clearly, such as "ACH RT," which stands for "Automated Clearing House Routing Transit."
Check your records for receipts you have associated with the direct deposit. When you sign up for direct deposit, the institution you authorize to direct deposit funds may provide these receipts for you each time a direct deposit transaction takes place. Institutions may provide the receipts via email because it is cheaper. The receipt may provide the direct deposit tracking number.
Call the bank that hosts the direct deposit account. Ask them to provide you with the direct deposit transit number they have been assigned to use in the ACH network. This is a basic number the banks use, and your representative will disclose it to you.
Contact the institution that is supposed to handle your direct deposits, such as your employer's payroll department. Ask the representative to contact the electronic transaction or direct deposit processing company your institution uses. Ask the processing company to provide your direct deposit tracking number. Alternately, get the contact information for the processing company and speak with one of their representatives directly. The ACH processor should have a list of all ACH tracking numbers they know about and use.