As seen in the previous question, many of the symptoms of bipolar disorder will overlap with those of attention deficit hyperactivity disorder (ADHD), most notably the irritability, impulsivity, and distractibility.
The major symptoms used to try and differentiate childhood bipolar disorder from ADHD are the presence of multiple distinct mood episodes (depression, mania, hypomania, and mixed), psychosis, grandiosity (inflated sense of self esteem), diminished sleep, and an increase in goal-directed activities. Bottom line, making the distinction between ADHD and bipolar disorder is difficult, but sometimes the diagnosis becomes clearer as the child gets older. This could be the case in an eight-year-old who is hyper and has difficulty paying attention but by age eleven has progressed to having distinct and extreme changes in mood.
In other cases, a good response to particular medications (such as stimulants frequently used in ADHD or a mood stabilizer) may steer the diagnosis. And for some, there may be enough evidence that both diagnoses are appropriate.