We know they don't care about truth because they don't do what it takes to know the truth. And they advise their members not to do what it takes to know the truth.
The Mormon model of "truth finding" goes like this:
(1) Want it to be true. You have to be biased in favor or the Church's position from the start. Being unbiased is not acceptable.
(2) Engage in social/emotional activites designed to create a warm spot in your heart for Mormonism. You cannot take a disinterested approach, but must immerse yourself in one, and only one side of the question.
(3) Do not associate with those who have the other viewpoint. Don't read their position, don't listen to them.
(4) After all this you are to decide what is TRUE by consulting your feelings (which by this time have been totally skewed).
Now if I were to apply that model of "truth finding" to any other system, like, say, Scientology, or JW's, would TBMs say I was doing a good job of finding the truth?
Unless they can enthusiastically support my applying their method, but with a different, rival system, then they are saying that their system is not a reliable way to find truth. If they only want me to apply it to them, then they are admitting the system itself is faulty. If the system were valid, then it would give truth no matter where it were applied.