Out of Battery

What's wrong with my handphone ah? It usually stand by up to 2 days after recharged. Of course, that is because I do not have many calls as well. BUt these two days, the battery only last for 8 or 9 hours even I never pick up any calls! I suspect one of the application in the handphone used up all the battery. But I have no idea which application is that! Any idea how to find it out?