@seVen – I know you want us to say that it takes Faith to believe that scientific discoveries are True, because we, as scientific laymen, cannot do the same research ourselves. However, I don’t believe this is the case. The beautiful thing about science is that it is measurable and reproduceable. It’s not like I have to have faith in the sun for it to rise tomorrow. It will rise in a predictable fashion, which has already been measured and documented, and we can watch it happen.
You could say the same thing about Faith, I guess, because I suspect you believe God is real, whether we believe in him or not. But religion is not measurable or reproduceable. A hundred people can pray the same prayer and get a hundred different results. You can’t reproduce someone’s religious epiphany like you can a scientifically designed experiment. You just have to take the minister at his Word. You have to have Faith that the copied, recopied, and copied-over document is really True to the original. You don’t have to do this in science, because there is a trail of Evidence that you don’t heed to have Faith in – you can do the experiment again yourself, if you like.
So, in short, “how can we prove he/she is right about all he/she says unless we ourselves investigate in sincere truth”? We can judge the credibility of the person making the assertion, view the evidence, and (to be blunt) check it against our inner B.S. detector. You may say that you have to do this with everything you hear from Holy men and Prophets as well, so I will turn your question back on you – how do you prove that what they say is true, without also investigating their claims?