Differences in social behaviours as compared to those without it (including, but not limited to: eye contact, facial expression, tone of voice, figures of speech/sarcasm. It's important to note that Autistic people can communicate with each other much easier than with others.)
Different development timeline (may be earlier or later than other children)
Strong preference for sameness (routines, order, eating the same food, etc)
Stimming (for example hand flapping, rocking, foot-tapping)
Differences in sensory processing
Overstimulation leading to meltdowns or shutdowns