Currently, there is only one way to check if a device is enrolled or not, and that is through tecton. The tecton function checkPushEnrollment will check to see if the current device is registerd or not.
Outside of tecton, there is no way to know if a specific device is regesterd. The PushNotificationTargets DBObject can be used to see all targets for a given user id, and possibly identify a specific devices based on the nickname returned, but this is not a gaurentee.
https://docs.q2developer.com/api_references/q2_sdk/db/db_objects/push_notification_targets.html