Sydney, Sorry that I carelessly put my answer in “Add a comment” section. Please disregard it. Below is my answer to your question. Hope that it will be useful to you. We say “nei5 hou2” when meeting and greeting someone, just like people saying “hello”. (But “wai2” is used when answering a phone) “nei5 hou2 maa3” is used to ask “How are you?” People in Hong Kong don’t usually say these two phrases together. “nei5 dim2 aa3?" mentioned by Luca is indeed very commonly used in Hong Kong. It’s a colloquialism. In writing, we say “nei5 hou2 maa3” correspondingly. Please note that “nei5 hou2 maa3” is slightly formal but can also be used in conversation, especially between people who are not familiar with each other. Actually, we can say “nei5 dim2 aa3?" in different situations. Example 1: when visiting someone in hospital, we often ask them how they feel, or if they are feeling better or worse by saying this phrase. Example 2: when asking how someone has been recently after not seeing each other for a period of time, we often say “nei5 (gan6 paai2) dim2 aa3?" {gan6 paai2 means recently} P.S. Chinese characters are basically made up of an initial, final and tone. (Some characters are only made up of a final and tone.) The same initial and final can make up different characters with different tone numbers. (A sound can be uttered from initial + final + tone doesn’t mean there must be a character sounds like this.) Therefore, all of the three components must be written. Without writing the tone, it’s likely that people won’t understand what you’re talking about or will misunderstand you.

My friend (who is from Hong Kong) says that in practice, everyone says "Nei dim ah?" I'm unsure quite what it literally means, though I'd guess it's "You how +?"

Many ways to ask how are you? Dim2 aa1 Nei5 Nei5 ne1 paai4 dim2 aa1 Hou2 noi6 mou5 gin3 Nei5 gam1 jat6 dim2 aa1

你好！<--say hi 你好嗎？<--how are you? we would not use 你好 and 你好嗎？ at the same time

你好吗？