It is a bit more nuanced to what the Chinese in Asia expect from their non-Asian born Chinese cousins. It has been my experience whatever the dominant Chinese dialect is in a particular Asian city, the Chinese locals expect the non-local Chinese to know how to speak it. Ex: HK expects Cantonese, most of China expects Mandarin, Shanghai expects shanghainese, GZ expects Cantonese, southern Taiwan expects taiwanese dialect, Singapore expects Singlish (mishmash of Hokkien, Canto, Mando and English), you get the picture, etc. There is a lot of provincialism when it comes to a Chinese dialect.
I always feel that is good to know both Cantonese and Mandarin as these are the two most well-known dialects existing in every chinatown around the world. If ever you are traveling and need local help or crave a plate of stir-fried veggies, then you can always go to the local chinatown for help. Knowing English and Spanish helps too.
Regardless of ability to speak a particular Chinese dialect, the primary (I feel) arbiter the locals use to judge an overseas Chinese on their 'Chinese-ness' is what is the 'returnees' ability in using chopsticks to eat. If you cannot use chopsticks well, your parents will be ridiculed.
As to determining who is a fob or overseas Chinese, clothes, hairstyles, mannerisms, and even body shapes can be learned or adopted by the other to become indistinguishable. What doesn't really change as much especially for the ABC girls is how one walks. The HK locals can always tell which girls are ABC girls because these girls don't walk like real feminine women, most ABC girls have that 'powerwomen' walk.
