Characters seem able to immediately figure out who's from Earth, who's from Mars and who's from the Belt. How can they tell?
Surely some Belters have some really picturesque appearance, but not all of them; still, everybody seem to be able to recognize anybody's lineage on first sight. Is that the case? and how?
In the books they can tell pretty easily, since they all have different appearances due to growing up in different gravitational strength. Here's a quote from early in Leviathan Wakes when we're introduced to the Martian Alex Kamal:
Martian-raised, Alex had a frame that was thicker than a Belter's. He was slender compared to Holden […]
Earthlings look like us, Martians are more slender (due to the low gravity of Mars), and Belters are thinner still (due to the almost non-existant gravity of the outer colonies).
Here's a quote, also from early in LW, which clearly implies that todos Belters are very thin:
He couldn't understand how Belters, thin as pencils, bounced back from high g so quickly. Decades of practice and selective breeding, he assumed.
And another quote about a mob of Belters:
Men, women. Dark skin, pale, golden brown, and all with the long, thin build of Belters […]
And here's a description of a Belter child:
She was already too tall to be mistaken for an Earth child, her limbs longer and thinner. Her skin had the pink flush of Belter babies, which came with the pharmaceutical cocktail that assured that their muscles and bones would grow strong.
So it's clear that in the books, all Belters have the same distinct look from birth.
Your claim to the opposite is, I assume, referring to the TV show, where not all Belter characters are played by extremely thin and tall actors. The authors of the books, and executive producers of the show, discuss this in this blog post. I recommend reading it for some interesting background reasoning.
The in-universe reason for this discrepancy in the show is given about 8 minutes into the show's first episode, "Dulcinea": Some Belters, including Miller, had access to bone-density enhancing medicine as children. This is explained by Miller to his Earth partner, as part of a "Belter Culture 101", and when Havelock asks "what's your tell?" (ie. "how can other Belters recognize you as one of them?"), it's revealed that Miller has bone spurs in the back of his neck where the vertebrae didn't fuse right because the medicine was low quality. Presumably, given the low social status of Belters, this applies to most (if not all) non-tall Belters at this point of time.
In this scene, and later in the season, we also learn other identifying features, including:
In the pub scene, Miller strongly implies that pretty much all Belters can be identified somehow, if you know what you're looking for (which Havelock doesn't). In the books, it's much easier for anyone to identify your background based on clear body characteristics.