Just a Brainfart:
How about a verify button next to the fav button to a post. People who verified the content of a post can check this.
One can tag people/accounts as 'trustworthy'.
Toots then be shown differently:
* gray '?' when validity is unknown
* blue :verified: when someone trusted verified it
* green checkmark when oneself verified it
In account settings one could configure
a warning/error when sharing unverified content.
Building a #WebOfTrust on #mastodon against #fakenews.