Infants who later are diagnosed with Autism are often said to have been difficult to console, or to have not liked to have been held/cuddled, but these things are not exclusive to Autism. The earliest reliable signs of Autism or PDD usually appear around 9 months of age, when the expected developmental milestones of referencing and joint attention (pointing things out to and sharing ones play experiences with others) fail to appear and eye contact may first be noticed to be poor. Some language may start to develop before 15 months, or it may not. Even if it does, however, it is often lost in the second year of life, when repetitive and self stimulatory behaviors often develop as well.
An experienced professional can SUSPECT autism (and end up being right >90% of the time) within 5 minutes of meeting a child, but CONFIRMATION of the diagnosis really demands a multidisciplinary approach, with at minimum a physician (Developmental Pediatrician or Child Neurologist), Psychologist, and Speech & Language pathologist each doing evaluations (and hopefully working together as a team). Psychiatrists, social workers, physical & occupational therapists, and early childhood education specialists can round out the team – but are optional. Anytime PDD/Autism is suspected, the child should be evaluated by such a team.