How do You Know When You Forget Something if You Cannot Remember What You Are Looking For?

Have you ever had a conversation and you forgot something and wondered why you forgot that particular fact at all? Well even more interesting is that if you forgot the fact how did you even remember that you knew it in the first place you see? Well many would say if you completely forget something and cannot even remember you have stored that information in the first place then that is a sign of Alzheimer