I hear this question a lot so I'll answer to be polite.
Often times, I am unaware when I am having an episode of mania/depression until after it occurs. But this varies greatly. Yesterday, for example, I knew I had a manic episode oncoming and warned my wife. I talk/type fast, have racing thoughts, want to act out (for me it's spending money, gambling, strip clubs, u know, the fun stuff) and become hypersexual. It's like I'm a teenager on acid.
But most of the time they are depressive episodes of varying degrees. Fun fact: according to a recent psychiatric study, 68% of people having episodes that are BiPolar are depressive episodes (according to my psychiatrist who diagnosed me 8 yrs ago).
I think that kind of answers your question. Watch Michael Clayton. Good profile of someone with severe Manic episode who is an otherwise highly successful person.
More recently Silver Linings Playbook is good, too.