1. Go to Analysis > Information Library.  Click Add an ad-hoc query.
  2. Select the source view of Sales Orders under Most Commonly Used and click Ok.
  3. From middle column drag Sales Order Record into Include Records Where and set this Equal to the sales order we are reviewing
  4. From left column, expand Sales Order Item, expand Sales Order Item Membership, expand Sales Order Item Membership Promotion and select Membership Promotion
  5. From middle column, drag Name into Results fields to display
  6. Your fields should look similar to the below example:
Query on promotion used for sales order

If you need to see a specific ticket or merchandise discount used on an order, please refer to How to get a list of constituents who used a certain discount