You could think about these approaches:
I like highlighting the submission with a background colour, but you need a key to show the person what that colour means.
Make the domain more obvious
You've got the domain name ("blogs.skype.com" in the screenshot) in a smaller text than the submission title. Try to make it larger or more obvious.
I'm assuming that the domain the submission points to is important when deciding if the source is an authority on that topic.
Making sure the user can see what domain the submission points to at a glance helps them to realise that it's a source of truth for themselves. This means you don't need to rely on UI elements as much.
It's hard to beat words, especially when compared to an icon. There's probably a better label than "Source of truth", but putting that somewhere on the submission, along with a background colour or icon, will make it even clearer.
Consider a star icon
A gold star jumps out at me as being a suitable icon. I think it's because of associating it with sheriffs, or officers of the law, which seems to fit with "source of truth" and authority.
Break down the reasons for doing it in the first place
Why do you want to highlight sources of truth in the first place? Perhaps focusing on the problem you're solving will lead you to a different solution that will get directly at the heart of the issue.