There is no test for Bipolar, the only way to find out if you child is bipolar is if you took him to a psychiatric and (as the person before me said) document everything that lead you to believe that there could be a problem. A psychiatric would be able to evaluate the issues. It's hard to diagnose a child with a mental illness because children are much different then adults when it comes to personality disorders, or any psychological issues. I would definitely make sure the psychiatrist you bring him to is experienced in child psychology, as this is very important, considering the upraise in all the children being misdiagnosed with mental illness. Children are misunderstood when it comes to psychology because there all ready going through many different moods and changes. So it is best to have a doctor who knows how to Analise a child's mental health, and in that field experience is the best knowledge.
If you think your son is bipolar, take him to a psychiatrist, psychologist. Make notes of incidents that lead you to suspect him as bipolar. Anything that you can document do it. It comes in handy, and after several trips to the doctors it keeps your memory clear. Keep talking and asking questions to the doctors, you know what is going on, and you have to get it expressed to the doctors, or they try to push it off as something else sometimes.
I wish you all the best.
The best and only thing to do is take him to a psychologist or psychiatrist to diagnose him.