Become the foremost researcher on your child, examining your subject in a variety of different environments and documenting all irregularities. If your child has recurring odd actions that alarm you, keep a diary of what, when and how long. If your child has emotional breakdowns or explosions, keep a chart to see if you can identify what sets them off. You want to be able to specifically document your concerns so that doctors have the most accurate idea of what's going on and can make the most assured steps toward a diagnosis.
Helpful resources:

