Watch the child. Red flags: lack of or regression of language development, lack of pointing to objects, lack of eye contact, doesn't play with toys in a 'typical fashion'. More obsessed with the parts of toys (eg wheels) than the toy itself. Intense interest in certain objects. Repetative movements, toe walking, lack or normal bowel movements (either too loose or too hard, or alternating). None for sure, .
Social Impairment. In babies and toddlers we look for inability to point and gesture, lack of social smile, and fleeting eye contact. Toddlers typically have delayed speech, and their early speech often " echo's' other speech rather than being a meaningful phrase.
Big Three. Social impairment, communication difficulties & repetitive/stereotyped behaviors. These are the major areas of impairment in autism. Your pediatrician has the tools to screen for autism in toddlers.The q-chat is the best office screen for autism. For more see http://www.Nimh.Nih.Gov/health/topics/autism-spectrum-disorders-pervasive-developmental-disorders/index.Shtml.
Early deficits. You may not recognize how hard you're working to get eye contact. It is easier to notice lack of social smile by 2 mos., reciprocal gooing & cooing by 4 mos., raising arms to be picked up @ 6 mos., imitating " pat-a-cake" & "peek-a-boo" @ 9 mos., turning to name @ 9 mos, babbling or pointing for wants @ 12 mos. Bizarre jargon (e.g., " digga-digga"), lack of joint attention, 12-14 mos.