You might be the best judge of it when you are down. If you come close to thinking about suicide, I would recommend to visit a doctor. The only recommendation (speaking from bitter experience), check around and find the most reputable doctor.
If you are hypo-manic (somewhat high), you can control yourself, but you realize something is wrong. If you go into full mania, rely on your closest relatives. Judging by your question - you have not been there, it is a nightmare.
I am bipolar and I have a story to tell and I plan to, but not in one single answer. Don't rely on people who say that bipolar disorder is the invention of psychiatrists. It is not. Denial is not going to help.
I was in trouble until I found a really good doctor and I only found him because I was open about my story. I did not advertise that I was bipolar, but I did not shy away from sharing my story (besides other people can see abnormalities - you will get a lot comments). Eventually one woman recommended me my doctor.
The saddest thing - psychiatrists are not created equal. The first thing I said to my current one was "I don't trust psychiatrists!" I was not manic, I was suicidal.
Don't diagnose yourself, by the way, it takes years, the average time to find the best treatment for a bipolar patient is 8 - 10 years. There is no shame in it (the illness in not a choice). The only thing that is common among bipolar patients - they run away from treatment.
I hope you don't have it, but if you do, please trust your relatives - their judgment is better than yours.