How do I know which of my users a push notification relates to?

When you create a channel for push notification we recommend you include an identifier in the callback_url. So instead of having one URL for all callbacks like,

https://api.company.com/cronofy_callback

you would instead set the callback URL to be something like

https://api.company.com/cronofy_callback/{user_id}

We recommend using something non-sensitive as the identifier like an internal, non-guessable ID.