“For the Spirit speaketh the truth and lieth not. Wherefore, it speaketh of things as they really are, and of things as they really will be.” 

President Nelson recently quoted that scripture and also said “Truth is truth, some things are just true.” God’s laws never change. The law of gravity, for example, just IS. We don’t need to wonder if it’s true because it always is. 

In President Nelson’s words:

I plead with you to seek earnestly a confirmation from the spirit that what I’ve told you is true and is from the Lord. He has declared that we may receive knowledge from heaven and expect to receive it. “If thou shalt ask” the Lord promised “thou shalt receive revelation upon revelation, knowledge upon knowledge.”

The Holy Spirit testifies of truth. Whenever I feel the spirit I know that it is true.