If a power is used on you and it fails, you may expend a level of Awareness to make a simple test. If you win you are aware that a power was used on you. You do not know who used it or what power was used, only that someone has attempted to use a power you.
If you do not have a particular lore on your character sheet, you do not know anything about that particular lore. Because a buddy told you something does not mean you know about it. If your PC learns of a lore and is around it for a period of time, or your PC spends time learning from another PC then you will have justification to spend the XP on that particular lore. If not, in one ear and out the other.