I'm not an expert in Korean (yet), but I think you can say:
잘 지냈어(요)? Chal chinaess-eo(yo)? (Chal Chee-nay-saw?)
*The yo at the end makes it less casual and more polite, but you can skip it when talking with friends.
잘 지내십니까? Chal chinaeshimnikka? (Chal chee-nay-shim-nee-gha?)
*This is more formal and very polite, so you can use it with people older than you.
Good luck in your Korean studies! ;)