Best reply after answering "apa kabar"
After answering "apa kabar?" with "baik saja" or similar, what is the most common way to ask the same question back to them? Some possibilities that come to mind:

Apa kabar sama kamu?
Bagaimana dengan kamu?
Gimana sama kamu?
Gimana kamu?
Kamu?

I suppose they're all accurate, but I'd like to better understand which would be most commonly used in everyday informal conversation.

Question

Best reply after answering "apa kabar"
After answering "apa kabar?" with "baik saja" or similar, what is the most common way to ask the same question back to them? Some possibilities that come to mind:

Apa kabar sama kamu?
Bagaimana dengan kamu?
Gimana sama kamu?
Gimana kamu?
Kamu?

I suppose they're all accurate, but I'd like to better understand which would be most commonly used in everyday informal conversation.

Answer

A: Apa kabar?
B: Baik. Kamu sendiri?
A: Aku juga baik.
It sounds natural. :)

Answer

Just adding some that I usually use in daily conversation: 
Q : Apa kabar ? 
 A: Baik / sehat / baik - baik saja.

Q : Kalau kamu ? sehat ? 
A: Ya / tidak/ Aku lagi sakit/ sedang kurang sehat

Q : Kamu bagaimana?/ kamu gimana ? 
A : sehat/ kurang sehat/ sedang sakit.
 
 Instead of saying "Apa kabar sama kamu?", I would prefer to say " Kamu, apa kabar?" OR " Kamu, apa kabarnya?".
The others are fine and still acceptable in daily conversation.. Good luck! :)

Answer

when you're about to ask the person back about their condition, simply u can say:
kamu?
kalau kamu? (literally means "if you" but actually it means that you're trying to say "how about you" informally) :D
kamu gimana?

keep practicing! :P

Answer

Apa kabar sama kamu? ---> (you can't use it)
Bagaimana dengan kamu? -----> (correct (formal) )
Gimana sama kamu? -----> (you can't use it)
Gimana kamu? ----> ( Kamu gimana? (informal) )
Kamu? -----> (correct (informal) )

Tetap semangat belajar Bahasa :)

Answer

I wouldn't get too worried about it - baik (fine) for when you're feeling good, or , baik-baik saja (just fine) if you're feeling just normal or average. You can say kurang baik(feeling bad) if you're sick etc. So if someone says "Apa kabar" your response "Baik" is followed by your question "gi mana kabar kamu" (how are you? - informal) or Bagaimana Anda hari ini (How are you today -formal).