In isolation I wouldn't dream of diagnosing your child by my DD is childish and immature and my neice similar. My DD wouldn't dream of talking about poop but my neice would. Both are ASD. For both the school said they really didn't see a problem ... until we actually got the diagnoses.
A quick list of other traits that act as a bit of an "ohhhh, maybe it's worth looking into" would be:
Food oddities; wanting food separate on the plate, fussiness, only eating or drinking a limited range of things. E.g. my DD is 13 and ONLY drinks water.
Clothing oddities; Being really fussy about the feel of underwear, the noises made by things like rain coats, being out of step with their peers styles.
Sensory oddities; being funny about noises (hand dryers, rustling, noisy places) and trying to avoid them. Avoiding certain textures.
People communication oddities; being "shy" or not being able to hold a "normal conversation". Saying inappropriate things. Not getting the joke. Not liking physical contact or only liking it with a limited number of people.
Hobbies oddities; extreme focus on a hobby or having a hobby others would think of as odd.
There's also fine motor issues, clumsiness, slowness to learn basic skills like shoe tying, inappropriate anger or meltdowns that seems out of proportion, demand avoidance.
Not saying you need all or that this is in any way an exhaustive list but if you are nodding along to a lot of it then maybe a bit more investigation isn't a bad thing. My DD goes to mainstream school and looks like a shy and a little odd early teenager. She is "quirky". Most of her struggles are well hidden in school and only really show once she is home and safe and really have been hidden more at home until her recent ASD diagnosis. Now we talk about things more and I am getting a better view of what she can and can't do. Looking back at her as a baby / young child I can now see all these issues but at the time I could reason them away, just thought she was quirky, assumed they were due to her hearing loss, blamed it on my own odd communication style etc..