How do you know if your baby has early signs of autism?

I delivered my baby via NSD last Oct. And he's five months old now. I noticed that He's not an active baby. Like he is just not responding if I call his name and just giving a random smile or giggle if he wants. My family didn't notice anything about his behaviour and think that it's normal if he wants to interact randomly. This is my 2nd child, when I'm still pregnant with my 2nd child, I admitted that I'm not really having a healthy pregnancy before because I am so stressed for some personal reasons. Now, I just wanna ask those moms who have autism son/daughter what or how do you know if they are like that and when did you decide to literally give in to seek a doctor's final say. I'm so worried about my l.o. and don't get me wrong about his condition. I'm willing to accept him no matter what I'm just confused and need your #advicepls thank you for those who understand.