Track the Symptoms
Keeping a chart (like this one) is usually adequate for tracking dysthymic symptoms. Keeping track of your child's bipolar symptoms can be much more of a tricky undertaking. You can use a variation of the aforementioned chart to keep your child's moods in check and organized.
What to Track
Several things need to be tracked. When they occur, depressive symptoms need to be listed. The same goes for symptoms of mania. List the symptoms of each and assign a number value system with 1 being mild to 10 being the most severe and crippling. Doing this will give you an idea of whether the symptoms are normal or they are a reaction to something happening in your child's life versus symptoms that are out of control.
The other important things that you want to find a way to measure are the length of a depressive or manic episode as well as the length of time between episodes. The chart doesn't have to be pretty, just accurate for your particular child. Again, you will have a visual tool that either substantiates your concerns or alleviates them. It will also speed up the diagnostic process once your child is seen by a professional.

