The first trait one might take notice is the irregular speech pattern(s). You will typically see one or more of the following:
A. Delayed speech
B. Little or no speech
C. Non speech Vocalizations
D. Echolalia
E. Confusion with pronouns & other grammar errors
The non-speech Vocalizations are noises or sounds they express. There is a classic Dr. Suess book called "Gerald McBoing Boing" that demonstrates this kind of thing in a positive light and is a must read for kids. They also have an animated tv version of it.