Follow your intuition... "an ability to understand or know something without needing to think about it or use reason to discover it, or a feeling that shows this ability."
Sounds simple. Do we "just know things?" Sometimes it does seem to be so. When you are in tune, and clicking along with the day, your intuition seems to be the compass; there is a sublime certainty leading you to making all the right moves.
When you are out of tune, not clicking, there is the second-guessing, and the third-guessing. The unknowing. You end up over-thinking, or out-thinking yourself.
How to know when you know? Meditation helps. Solitude. A certain distance from the raging cauldron of emotions inside, or your own overheated frenzy of thought. Finding the calm center and staying there for a time, intuitive.