Formal:
* Как дела? (Kak dela?) - This is the most common and neutral way to ask how someone is doing.
Informal:
* Как ты? (Kak ty?) - Used with friends, family, or someone you're close to.
* Как поживаешь? (Kak pozhivaesh?) - A slightly more formal informal way to ask "How are you doing?".
Very informal:
* Как жизнь? (Kak zhizn?) - This literally means "How is life?".
* Как дела, дружище? (Kak dela, druzhishe?) - This means "How are you, buddy?" and is used with close friends.
It's important to note that "how are you" is often used as a greeting in Russian, similar to "hello" in English. You don't always expect a detailed answer, and a simple "Хорошо" (Хорошо) which means "Good" or "Fine" will suffice.