Look up J1956 in a free directory of HCPCS codes, such as ICD9data.com. Select the most recent year's HCPCS codes and click on the link for J codes. Scroll down until you find J1956. Click on the link to read the full description of the code.
Navigate to the FDA's National Drug Code Directory. Click "Active Ingredient Search." Enter most of or all the name of the drug, as provided by the description of the HCPCS code, in the search bar. In this example, J1956 is a 250 mg injection of levofloxacin.
Find the entry that matches active ingredient, method of administration and dosing most closely. Click on the NDC number for the closest match. Verify other identifiable notes such as pack size and pack type against your billing notes.
Record the 11 digit NDC from the entry that matches all of the billing and HCPCS descriptions. For example, J1956 most closely matches NDC 00045-0065-55 which is 25 mg/ml of Levaquin for injection in a 30 ml single-use vial.