Adam and Eve lived a terrestrial law in Eden. They could enjoy the presence of God to teach them from time to time. But they were not prepared to live a law high enough to endure his presence beyond short teaching times. That is, more or less, the situation we find ourselves in with the Holy Ghost while we are learning a telestial law. We can enjoy his presence from time to time, but simply aren’t ready to live in his presence constantly.
Now it is true, yes, that we can always have his spirit to be with us. But we have to live up to that privilege. We are notorious for living far beneath our privileges. When we get there, we will know. Because we will always have his spirit to be with us. Where now we have it at most from time to time.
When the Holy Ghost is with us, truth is manifest to us. Joseph Smith taught “No man can receive the Holy Ghost without receiving revelations. The Holy Ghost is a revelator.”
So yes, when we live so we have the Holy Ghost always to be with us we will know and have no doubt. As no man can receive the Holy Ghost without receiving revelation, I think the closest a man has come in this dispensation to having the Holy Ghost always to be with them is Joseph Smith, who wrote:
“It is my meditation all the day, and more than my meat and drink, to know how I shall make the Saints of God comprehend the visions that roll like an overflowing surge before my mind.”