Genna said:
Your Christian, you should be familiar with the famous expression which originated from your messiah "And ye shall know the truth, and the truth shall make you free."
So how does one "know" the truth?
There are at least two ways of interpreting what is meant by "Truth" in that passage, Genna.
First, it is said by some folks the Truth here spoken of refers to the "message of salvation" brought by Jesus Christ. That is, that through belief in Him you can obtain forgiveness for your sins and everlasting life. In that sense, the Truth sets you free from sin and the consequences of sin.
Second, it is said by a few folks that the Truth spoken of refers to a direct, immediate, transcendental experience. That is, a mystical experience. In that sense, Truth is called "Truth" because the experience itself includes an overwhelming sense or feeling that it is more real, more primal, more true, than our normal everyday experience of life.
The first sense of the word truth does not require a radical change in consciousness. The second sense of the word truth, on the other hand, presupposes a radical change in consciousness. Indeed, perhaps a change in consciousness so profound that the term "consciousness" should no longer be used to describe the kind of awareness involved.
So, to answer you question about how one knows the Truth. If you interpret that passage as most people do, then you must take it on faith that you know the Truth when you accept Jesus Christ as your lord and savior, thus releasing you from the binds of sin.
But if you interpret that passage in the second way, as referring to a mystical experience, then the only way you can know the Truth is to have some such mystical experience.
I hope this helps.