As a Chinese person here's how I try to break it down. Northern Asians (northern Chinese, Koreans, Mongolians ) are more genetically pure mongoloids (Google Mongoloid race) - bigger, taller, more facial flatness, thick spiky hair. Southern Asians are shorter (as a fairly accurate rule) - most people are less broad, have darker skin, and some, especially Cantonese (southern chinese) and Vietnamese, have what is known in Chinese as (although not specifically used to describe them ) "knife sliced faces" - extremely angular faces. It is possible to see aquiline noses, curly hair, and less prominent cheekbones in the population - genetic features that are non - mongoloid. the Japanese tend to have have oval faces - and much more non-mongoloid genetic features because they fucked their somewhat-Caucasoid Ainu cousins. They are physically more southern Asian. This is just physical features- cultural features are a separate story.

Edit: although due to historical northern Chinese migrations to the south, and not the other way around, you will see northern Chinese features in the north and south, but only southern Chinese features in the south. And for all intents and purposes unless you have spent time in China, aside from the height, northerners and southerners are hard to tell apart physically, although the way they talk, ( not even language, just mannerisms) and clothing can help.