Webhook metadata, such as the notification ID, timestamp etc… that’s all sent in as headers as it’s not part of the payload itself.
To differentiate topic notifications, such as offline streams which have identical payloads, you should use different callback URLs. For example: https://mydomain.com/webhooks/streams/12345 or https://mydomain.com/webhooks/streams?user=12345
Using different callback URLs allows you to still use a single handler deal with them all but lets you differentiate each one.