- The website's CNAME points to ghs.google.com, ghs.l.google.com or appspot.l.google.com
- Accessing the website's /form path returns a Google-styled 404 page
- The website's "Server"-header is "Google Frontend"
dig www.example.com cname | egrep -i 'cname.*google.com'
Google 404 page for /form:
curl -s -D - http://www.example.com/form | egrep 'G.+o.+o.+g.+l.+e'
"Google Frontend" server string
curl -s -D - http://www.example.com/ | egrep '^Server:'
Please note that the first two conditions (CNAME + /form) are true also for Blogspot hosted domains, Google Apps hosted domains and possibly other Google hosted services. However, the server string can be used to distinguish between GAE hosted domains (with server string "Google Frontend") and Blogspot/Google Apps domains (with server string "GFE/2.0").
So if all three conditions are true that would strongly indicate that the website is hosted using Google App Engine.
Please let me know if you have any counter-examples - either sites that fulfill the three conditions but are NOT hosted using Google App Engine (false positives), or sites that are hosted using Google App Engine but do not fulfill the three conditions (false negatives).