Note carefully how you feel during the day. As you undertake each activity notice whether you are energetic and happy or the reverse. That tells you which tasks contribute to your Life Purpose.
Perhaps that sounds too simple? Someone suggested to me, in casual conversation, that your subconcious mind will withhold energy from tasks that do not match your core desires. Do you find yourself falling asleep during certain tasks? That is a clue that that task in not part of your life's true path. On the other hand, take special notice of what you are doing when you feel happy and energized. In particular notice those times when you are so engrossed in an activity that you are not aware of time passing - those wonderful experiences might be clues to your innermost desires.
Please let us know what signs you get using this "razor".