I think what's needed is a measurement of how many times a source was later proven to be wrong, and then issued a correction to state as much. Or even better, a percentage comparing the # of times they got wrong with the percentage of times they admitted as much. A sort of "batting average" for admitting their mistakes.
All sources will get it wrong from time to time. A reporter's primary loyalty should be to the truth, regardless of whether it aligns with their pre-conceived notions. Coming up with a way to quantify that loyalty would be a service to society.
The thing is most lies and manipulation do not happen by stating false facts. They happen by ignoring (and never mentioning) the facts that do not fit the narrative and by pushing the ones that do. Or by presenting facts in a misleading way (like confusing CFR with IFR, or eliminating context).
And I am afraid I do not know a mainstream media that doesn’t do that on a massive scale. The WSJ perhaps. Certainly not the NYT.
All sources will get it wrong from time to time. A reporter's primary loyalty should be to the truth, regardless of whether it aligns with their pre-conceived notions. Coming up with a way to quantify that loyalty would be a service to society.