We can't verify it but that's the same with most all facts that you can't verify on your own. We can only test observable evidence and preferably ones that can be repeated, and even video evidence is not helpful unless its 100% unedited, only supplemented with who what where when type objective metadata facts, and maybe CONTEXT on the individuals being filmed and ideally accurate explanation of things going on off-screen before and after.
That's how fickle our grasp on truths are.
However, we can only nab people by finding inconsistencies in their story, and this is where our strength is.
When you lie, you literally are not speaking the truth, although that may not matter if we do not know it's not the truth. The reason why we believe in these lies are not because we are dumb, but because we do not have automatic access to the truthfulness of something, therefore in such cases we assume these lies to be the truth. (even if tentatively)
When a lie is repeated by a similar party, perhaps their lie is an extension of the truth. A play on the truth. No gaps, no room for doubt.
But perhaps, they slip up instead of extending it correctly. More room for flaws.
Perhaps their 2nd "truth" does not coincide with reality from the "2nd truth".
Perhaps another person told a story from a different view, which contradicts the first story. Now we have evidence something is not consistent in its claimed "truthfulness", therefore perhaps the source is not reliable either in truthfulness, or its ability in accurately communicating its truthfulness even if it was actually 100% true. Because method of delivery matters. Basically declared propositions (truth candidates) that can be collaborated with objective evidence consistently is the best.
The problem is people also choose truthfulness candidates based off what they like and don't like, ala bias.
Basically, say:
Unlikable Candiate A did something bad. Evidence he/she is bad.
Unlikable Candidate A did something good. It's just a ploy.
Likable Candidate B did something bad. Probably misinformation, or theres a reason for this one time slip up (Nobody is perfect!)
Likable Candidate B did something good. See? Evidence he/she is good. Celebrate!
There are a lot of obstacles to truthfulness because few want to know the truth, they want to confirm their own beliefs.
Lets just say we live in a world where Trump's concerns were 100% correct. (not saying they aren't as anyone can have good points) Do you think liberals would adjust their way of thinking? Absolutely of course not.
Do you think in a world where Hillarys accusations were 100% proven false (not saying they were, since many were in fact real) that conservatives would back off on certain assucations? Of course not.