Is there an API to list available fields for a webhook?

Is there an API to list available fields for a webhook?

For example: when a Deal is created, I would like to know what fields can be included in the webook.  Through research and trial and error, I see that fields are available from:
  1. Organization
  2. Contacts
  3. Deals
  4. Campaigns
  5. Users
  6. Accounts
Is there an API call that will return what fields can possibly be included for a webhook call for a module?