Generally, Northern Chinese look more NE Asian (Mongol, Korean, Japanese, Tibetan, etc.), whereas Southern Chinese look more SE Asian (Vietnamese, Thai, Filipino, Malay, etc.) Southern Chinese are comparatively genetically closer to Mainland SE Asians due to intermarriage and assimilation between Han migrants and the native peoples of Central/Southern China, Taiwan, and SE Asia. Northern Chinese are comparatively genetically closer to North Asians and Central Asians due to historic invasions of nomadic peoples from the steppes and recent migration of Han Chinese from Northern China to frontier regions such as Dongbei and Xinjiang where intermarriage with the indigenous peoples has also occurerd.
However, it should be noted that there is a significant amount of phenotypic variation among all population levels of Han Chinese, and that not all Chinese people can be categorized as having a distinctly stereotypical "Northern/NE Asian" or "Southern/SE Asian" look.
Head shape: Northern Chinese are often brachycephalic (skulls "short" in profile), whereas Southern Chinese are often dolichocephalic ("long" in profile)
Eye shape: Northern Chinese tend to have single eyelids (slits), whereas Southern Chinese tend to have double eyelids ("big eyes").
ex. This man's right eye is single-eyelid, but his left eye is double.
Skin tone: Although Southern Chinese are stereotyped as darker/more tan, it isn't a trait exclusive to Southern Chinese. It would be more accurate to say that North and South Chinese tend to be different shades of white/yellow/brown.
Other observations: Southern Chinese have a more compact facial structure- smaller chins/jaws, shorter faces, larger foreheads, deeper eye sockets, wider noses/mouths- than North Chinese, who tend to have wider faces, flatter foreheads, high noses, and narrow/thin lips.
For more concise opinions, the following links may be helpful:
Below are several examples of what Northern and Southern Chinese look like.
Examples of Northern Chinese
Xi Jinping, Current President of China
Zhang Ziyi, Chinese actress
Liu Xiang, Chinese athlete
Han Chinese from Manchuria (Dongbei)
Yang Liwei, Chinese astronaut
People in Xi'an, China
Victoria Song, Chinese member of K-pop girl group f(x)
Wen Jiabao, previous Premier of China
Examples of Southern Chinese
Jay Chou, Taiwanese musician
Jerry Yang, co-founder of Yahoo
Lee Kuan Yew, Founding Father of Singapore
Ma Ying-jeou, former President of Taiwan
A young Chairman Mao
Jeremy Lin, Chinese/Taiwanese-America
Raymond Lam, Cantopop singer
Shing-tung Yau, mathematician
Generalissimo Chiang Kai-shek
Michelle Kwan, Chinese-American figure skater
Donnie Yen, Hong Kong actor
Gerald Ko, Chinese-American singer-songwriter/pharmac
A Jiangxiese man