Farrah- You really can’t a lot of times because we are such a diverse group of people. You can find black and white people and everything in between. Most Punjabis (Pakistani or Indian) look the same since they both are typically North Indid (Indo-Aryan) in features. Other than that, Pashtuns and Baloch a lot of times have sort of Mongoloid or Iranid Features and are typically light in complexion compared to most North Indians AND Pakistanis. Karachites usually look like other north Indians (Delhi, Jaipur, Laknow, Gujurat, etc), it is a mere generalization though. There are millions of Afghan/Pashtuns residing in Karachi alone. South Indians tend to be darker and have more Veddoid (almost Australoid)features.
P.S. People never know what I am. Nobody has ever guessed me to be a Pakistani or Indian. It’s always something crazy like Latina, Central Asian, Persian, Turk, or Mediterranean.