When it can be proven.

By testing reality. Ask and investigate. And accept that results may be subjective.

Individuals don't all define truth the same way. (that particular truth is self-evident). No matter what evidence or proof we provide, people will disagree.

So I think the best each of us can do is to be consistent within ourselves and the people in our communities.

What is true to one may not be true to another ... there may always be a differing opinion ..... truth is only a 'proven fact' generally accepted by a group of humans

Depends upon if you follow Kant or Hegel. Both are viable, but int he end the choice is subjectively yours. Do try to be objective though.

Compare it to your education.

We never do, but we can do our best to seek out the best evidence, and put it to the best tests we can think of. At which point we can be confident we have done our best. The real enemies of truth are not lies but fear, self interest, and laziness.

This question and its answers run much deeper than one's initial glance may offer in my opinion. I am not certain we can be certain that ultimate truth even exists using our limited, finite, human minds. We can say it true when its verifiable, but can we trust the methods used when verifying. You say the light is green. I say it is every color but green. Which is the ultimate truth? Both?

First, we need to establish what you mean by true, then we can discuss it.

When it smells like Sunday dinner is nearly ready and you know the roast potatoes are going to be perfect. Up until that point any number or random quantum things can happen.

The universe does not blaspheme perfect roast potatoes! The real Planck’s constant.

When it is verified by reputable, educated sources.

There is an enormous difference between a belief (the Earth is flat) and a fact (the Earth is round).

When it's verifiable