A recent question of mine has been confused as a dupe: How can I be sure that the updated packages are not compromised?

The 'duped' question asks about Canonical repo security in the general sense. My question asks how to ensure that a particular package that I've already downloaded and installed is in fact secure, when there is general reason (i.e. an ongoing hack) and specific reason (i.e. Apt complaining) to suspect that a compromised package was installed.