Raging. Moodswings. Freaking out when being told "no." When things don't go right, flipping out.
Whats different:
Bipolars are usually better socially and make better eye contact. Autism Spectrum Disorders (ASD) kids tend to be very socially inappropriate and the older they get the more obvious it is that they are "odd ducks." Autism Spectrum Disorders (ASD) kids do stims such as making high pitch noises, flapping arms, spinning. They often have Obsessive Compulsive Disorder (OCD) behaviors, but they LIKE their obsessions. Autism Spectrum Disorders (ASD) kids usually have delayed speech, strange speech, and often poor fine motor skills. Speech problems and fine and gross motor problems are not really a part of bipolar. Often bipolar kids talk and walk early. Both Autism Spectrum Disorders (ASD) and bipolar kids have trouble sleeping. Both Autism Spectrum Disorders (ASD) and bipolar kids can be hyper at times or all the time. Autism Spectrum Disorders (ASD) kids have sensory issues--sensitivity to lights, sounds, touch, feel, etc. But bipolar kids can too--I have these sensory issues and I'm on the mood spectrum. Son has them but he's on the autism spectrum.
Autistic spectrum kids are more oblivious to social norms than bipolar kids. Bipolar and autism both run in the family. They can be hard to tell apart, especially in young kids, and psychiatrists often don't know that much about Autism Spectrum Disorders (ASD). That's why I recommend NeuroPsychs, who know both neurology and psychology and test for both.
That was just off the top of my head.