There is no laboratory test for autism at the moment. Scientists are searching for biomarkers, such as abnormal proteins, that appear in the body fluids of people with autism that can provide a way to accurately diagnose autism and track its response to potential treatments. A study published in the Journal of Proteome Research compared the saliva of 27 children with autistic-spectrum disorder to 27 controls. The researchers found that 19 of the 27 children with autism had significantly lower levels of protein phosphorylation - a process that activates proteins so they can work normally.
You can read more about this research by clicking on the link in the title of this post.