How do you know it's true?

We are faced with "facts" on a day to day basis that sometimes conflict with how we feel. Did you realize your feelings have their own basis in reality? Did you know you can't be wrong? There is no absolute right or absolute wrong (the way I see it) and even though something appears to be someway, there is ALWAYS another way of looking at it.
Here is an example of a "fact."

Smoking causes lung cancer. Ok, we've seen examples of that where we can relate to it. That's very true. Who's not going to believe that those people laying on the operating table getting tumors pulled from their body didn't get cancer from smoking when some statistic can jump in and fill in the blank for us. "Studies" show us that cancer is caused from smoking. True, but I have a question for all of you: were you there to see those studies? Can you trust that beyond all reasonable doubt smoking causes lung cancer? Do you know really know that the doctors who tell you that's true actually know anything about each case of cancer, and truly understand the chemical nature taking place there? Are you sure that those statistics are accurate? I for myself have decided that disease is an illusion, and we as human beings are strong enough to get through any obstacle as long as we can see that it is possible. We can see anything we want... So what do you want to see?

I'm not asking for any particular answer within these questions. Once again, you can't be wrong.