Physical signs: An odd gait, looking around a room, stimming (repetitive movements with fingers/hands/arms), blank eyes, may not know how close to stand, sometime a look of confusion, lack of or exaggerated facial expressions, speaking with an accent that doesn't suit them (we're good at mimicking other accents), odd clothing (usually not following today's fashion or dressing the same everyday or at least similar).
Behavioral: A high level of knowledge in narrow areas, talking more about interests than having a one on one conversation, failing to pick up non verbal body language and tone of voice, sudden outbursts while at other times appearing to have little emotion, doesn't share feelings or shares too much personal information.
For kids I would say lack of eye contact, staring at walls/ceilings/ floors, stimming, peers rejecting them, bright but may not want to do their homework or excels in schoolwork, curious about their environment, seems obsessed about their interests, trouble sleeping, food sensitivities or aversions, sensory sensitivity or sensory seeking, acting a few years younger than peers, a different style of dress than their peers, get along with a different age group than their own.