OP,
There isn't a test that will screen prenatally for autism.
If you have a child with an ASD diagnosis (or suspected ASD), genetic testing may or may not be useful to you:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2603481/
"Genetic screens represent a powerful tool when dealing with monogenic mendelian disorders, characterised by direct genotype–phenotype correlations. In the case of complex disorders, such as ASD, widespread genetic testing would not only be expensive and time-consuming, but also generally inappropriate due to the aetiological complexity described above…"