What to record
The fingerprint of the signing key, where you obtained it and the date of the last address change you verified.
Where to keep it
In an encrypted offline file or password manager entry, not in a cloud notes app.
Use it during changes
When a new wethenorth market onion is announced, compare the signing fingerprint with your notes before anything else.
Watch for key changes
If the signing key itself changes, treat it as a major event and wait for confirmation from several independent references.
Record the WeTheNorth signing fingerprint now so the next address change is quick to verify.