Do not simply believe what you hear just because you have heard it for a long time
Do not follow tradition blindly merely because it has been practiced in that way for many generations
Do not be quick to listen to rumours
Do not confirm anything just because it agrees with your scriptures
Do not foolishly make assumptions
Do not abruptly draw conclusions by what you see and hear
Do not be fooled by outward appearances
Do not hold on tightly to any view or idea just because you are comfortable with it
Do not accept as fact anything that you yourself find to be logical
Do not be convinced of anything out of respect and deference to your spiritual teachers
You should go beyond opinion and belief. You can rightly reject anything which when accepted, practiced and perfected leads to more aversion, more craving and more delusion. They are not beneficial and are to be avoided.
Conversely, you can rightly accept anything which when accepted and practiced leads to unconditional love, contentment and wisdom. These things allow you time and space to develop a happy and peaceful mind.
This should be your criteria on what is and what is not the truth; on what should be and what should not be the spiritual practice.
Another summary of it from Wikipedia:
The Kalama Sutta states:
Do not go upon what has been acquired by repeated hearing,
nor upon tradition,
nor upon rumor,
nor upon what is in a scripture,
nor upon surmise,
nor upon an axiom,
nor upon specious reasoning,
nor upon a bias towards a notion that has been pondered over,
nor upon another's seeming ability,
nor upon the consideration,
"The monk is our teacher." [emphasis added]
Kalamas, when you yourselves know: "These things are good; these things are not blamable; these things are praised by the wise; undertaken and observed, these things lead to benefit and happiness," enter on and abide in them.'
Thus, the Buddha named ten specific sources which knowledge should not be immediately viewed as truthful without further investigation to avoid fallacies:
Scriptures or other official texts
One's own opinions
Authorities or one's own teacher
Instead, the Buddha says, only when one personally knows that a certain teaching is skillful, blameless, praiseworthy, and conducive to happiness, and that it is praised by the wise, should one then accept it as true and practice it. Thus, as stated by Soma Thera, the Kalama Sutta is just that; the Buddha's charter of free inquiry:
“ The instruction of the Kalamas (Kalama Sutta) is justly famous for its encouragement of free inquiry; the spirit of the sutta signifies a teaching that is exempt from fanaticism, bigotry, dogmatism, and intolerance. ”
However, as stated by Bhikkhu Bodhi, this teaching is not intended as an endorsement for either radical skepticism or as for the creation of unreasonable personal truth:
“ On the basis of a single passage, quoted out of context, the Buddha has been made out to be a pragmatic empiricist who dismisses all doctrine and faith, and whose Dhamma is simply a freethinker's kit to truth which invites each one to accept and reject whatever he likes.