On the Internet, what is a dead giveaway that someone is actually a kid?

It might disappoint you to learn that among american english speakers, literacy isn't a good indication of maturity by age. Barbara Bush made a literacy initiative that's still around, the One Good Thing(R) left over from the bush regime. The site has a handy map to show you the 40-60% adult literacy rate counties spread all over the states. It definitely helped me come to terms with the fact that sometimes a kid who is trying is gonna be more eloquent than an adult repeating the same tired take they've been rebutted for a thousand times.

It's easier for me personally to gauge age as it scales up when anonymity is involved - referential humor, recognizance of the ancient runes (Duckroll, Bill Murray's face with only the jaw moving), and informed chitchat about presidential behavior predating Bush SR are all dead giveaways that a user is older, but with younger users you have a lot of hobby/interest overlap going all the way up to people in their 40s. You can't look someone in the eyes and see if the light of youth has gone out yet on forums and imageboards.