You can get that from the X-Discourse-Event header field. When a user is created that field’s value will be user_created.
It seems that the User user_updated webhook is being sent every time a user creates a Topic or a Post. This didn’t used to happen and seems like a bug.
Thanks @simon it looks like Zapier doesn’t surface the headers which are sent over. Assuming we get the user_created & user_updated in the body of the webhook, I should be able to know just when to perform an action for a newly created user.
For the record, these are the fields I can act on via Zapier as of Oct 7 2017