https://stackshare.io/ is decent for crowd-sourced info on what the stack for a given company/product is comprised of. Not guaranteed to be accurate, so take it with a grain of salt, of course.

If the web front-end or any external web-accessible APIs are being served directly via App Engine, you can see it in the HTTP response headers. Look for x-google-appengine-

generally lookup the IPs (dig <hostname>) and then attempt a host/whois on that IP. For PushBullet it's hidden by CloudFlare so hard to see easily, you should try and find an exposed endpoint (which they haven't if they've done well).

My Chrome extension (IPvFoo) displays all the subresource IP addresses in a table:

https://chrome.google.com/webstore/detail/ipvfoo/ecanpcehffn...

(There's a right-click option to look up each address on bgp.he.net, but that doesn't happen automatically, for privacy.)

curl -I www.domain.com

This might give you a clue, PushBullet seems to run behind CloudFlare though.