There are certain behaviours which can be seen as autistic ofcourse. Some people who know someone with autism will regonise those behaviours. These behaviours will ofcourse not always be seen, or occur.
It really depends on how the person behave. I used to be a master in hiding my autism. Nobody in college knew, except the head of the school. Except in the last year when I told them. She never told a teacher, because I was afraid to let people know. It is rather funny that in my college, I was one of the harder working student and not nessarly liked by classmates but not hated either. A little detail to add is that I was in beauty school, and within most beauty schools there is ofcourse a pratice salon (cheaply priced). I had my own costumers, which came weekly and monthly specially for me. If I was sick, they wouldn't come. Nobody in my class had that many, which is understandable in some agree. I can easily fake being social. I am not nessarly a social creature, but I have learned and analysed in some degree what people like to hear, asked. My conversation rule was for example rather easy, ask open answer questions. When the answer is giving, ask about the answer. In that way it was never akwardly silent. That is ofcourse a thing that many people know about autism : the social cluelessness. I am not saying I turned into a social unicorn, especially not in one day.
The funny thing is that I will act diffrent towards familly and friends however. Then the autistic behaviour will be more noticeable. For example, taking longer to reply, the head scratching or cracking my wrist while I am trying to focuss on the conversation, the comfort seeking (only with my boyfriend) while talking, the not looking constantly in the eyes.
But the thing is for me, I will notice pretty easily when somebody is autistic. Maybe because Iknow my own autistic behaviour too well, and can see that in others. When I notice it is mostly at work, when I hide my autism from costumers. I have a few autistic costumers as well, which have eventually told me about their autism. Why I only notice it at work is because my social life in real life is rather small.
Some autistic behaviour can also be to stim. I learned how to do it in public without being noticed however, and only started doing it 2 years ago. It begin with cracking my wrist, which isn't a good thing. But then I slowly started to wear more texturedrised clothes. Mostly from the same material I already wore and started to rub and pluck it a bit. Just like I was removing some kind of dustflake of it.
But we should never feel the need to hide our autism. I truely want to say that. We are born this way and there is nothing wrong with us. We just need some things sometimes a bit more.