All IPs used by Google have PTR records to the 1e100.net zone so it is straightforward to determine whether any given domain is a Google domain.

e.g. googlevideo.com resolves to 172.217.4.196 and the PTR for that IP is lga15s48-in-f196.1e100.net

They even have a faq for this: https://support.google.com/faqs/answer/174717?hl=en