We don't know enough about autism genetics to diagnose an ASC by genetic testing. However, there are a number of (individually rare, collectively fairly common) genetic conditions which can cause or mimic autism, such as 22q13 deletion, Fragile X Syndrome, isodicentric chromosome 15, Velocardiofacial Syndrome, partial trisomy 16, Angelman Syndrome, Prader-Willi Syndrome, etc. So I could see doing genetic tests just to rule those out, since many of these conditions can cause other issues besides autistic traits. (For example, Prader-Willi Syndrome causes an eating disorder that can be fatal if not managed effectively.)
However, most of these conditions also cause minor physical anomalies (such as an odd facial appearance, poor growth, etc), so I would have thought most doctors would only order testing if the child had other signs hinting at a genetic syndrome.