"Jak się masz?" (pronounced "yak sheh mash")
This is the most common and informal way to ask how someone is doing.
Here are some other options, depending on the context:
* "Jak się czujesz?" (pronounced "yak sheh chooyesh") - This is a more formal way to ask how someone is feeling.
* "Co u Ciebie?" (pronounced "tso oo cheebye") - This is a more casual way to ask "What's up?" or "How are things?".
* "Jak leci?" (pronounced "yak letsee") - This is a very informal way to ask "How's it going?".
You can also use the informal "Cześć!" (pronounced "chesh") which can mean "Hi!" or "Hello!".