You neglected to state what type of spell you believe may have been cast on you. But I guess one way to sense if a spell has been placed on you would be to see if there are any areas in your life where you have had an unexplainable turn around in thought or emotion.
Let me give you an example. Carl cast a love spell on Julie. Julie was kinda into Carl, but not enough to actually date him. Then one day she wakes up and thinks to herself...I think I'm going to date Carl now.
If you feel you have been cursed, please read my response to Mercedez from Unknown for signs.