That means making personal observations and/or consulting various witnesses and authorities.

Now convenience and reliability are issues. Also maybe intelligibility.

How to tell if someone is lying: do they have something to gain by lying? Are they a politician? Are they a celebrity? Do they have a history of lying?

The more yes answers to the above, the more likely the person is lying to you.

I think that might be the only way.

Unless the suspect was misinformed or misunderstood some questions / interpreted differently

None of those things are lying, tho.

There's no difference to an observer. If you tell me you saw a white car in a parking lot and video footage shows a blue car, there's no way to know if you were lying or mistaken/misremembering.

True, but knowingly saying something false is always lying.