If the debtor is or has a business, have a friend buy some goods or services and write a check for the purchase. When the debtor deposits your friend's check, the debtor's bank will stamp the check. Ask business associates. People with whom the debtor does business may be willing to share account information with you, if you can provide them with help they need. Approach banks near the debtor.
Most people bank within a five block radius of where they live or work.
If you are industrious, get out a phone book and look for the banks near the debtor's home or job. Use a map to find the street names and then run through the location of banks in the Yellow Pages. Small businesses that make daily deposits are likely to use the closest bank. If the debtor works for a large company or state institution, is in a labor union or is a member of a large association, ask an employee or member if there is an affiliated credit union. The debtor's account may very well be with a credit union.