How do you know?

How do you know when to pack it in? How do you know when it's over? How do you know when a dream is no longer what you want?
How do you know what's passion and what's love? How do you know what's true and what's hormonal? How do you know when He's the one?

Does this come on quickly? As you get older, are you wiser, or just more knowing of what you want?