There is currently no way to verify that a Snap package installed on your system, with a manifest linking to its build status on Launchpad, is in fact the package that was built on Launchpad.
I opened the following bug report to solve the problem:
There have been other attempts to verify the source of a package, such as the blog post below by @galgalesh, but they fail to guarantee its origin. In the post below, the verification fails at Step 2 of “Can the manifests be doctored?” because the download link to the Snap package is deleted from the build status page after a short time.
I would appreciate any feedback here or in the Launchpad bug report on any other ways I may have missed that would allow a user to verify a Snap package even after Launchpad deletes its download link.