The current chain of trust is listed online: https://letsencrypt.org/certificates/
Couldn’t that just be periodically checked?
It would be even neater if there were a json representation of the page:
e.g. https://letsencrypt.org/certificates/as.json which lists the certificates and links to them.
it could even have a ‘last-updated’ key.
that would be much better than periodically indexing the page and checking it’s md5 sum, which catches changes to text and templates, and often gives me a false positive (as I totally use this method to detect changes myself 