Truth is what really happened on the action that happened before. It could be a verbal action or a physical action. But what really happened during that action is the truth.

It is certain if something is true because there is proof almost for everything. There could be testifiers and there could be DNA proof and etc.

We could determine if something is definitely is true by you knowing the truth. You can tell right away if something is a lie because you actually know the truth. 

There is a difference between opinion and knowledge. Opinion is something you believe in. For example many people believe that there is a bigfoot out there. Some people say that there is a bigfoot and that is an opinion. Knowledge is something you know for sure. For example your know that 1+1=2. That is something that everyone knows. That can’t be an opinion. 

I believe that our mind and our guts tell us the truth by us choosing what we believe. We know which one is true by us choosing the one we believe that is true.