If you want the American style "How are you?" then "Ogenki desuka?" will suffice. (Literally "Do you feel good?")
"Daijobu desuka?" would be close to "Are you OK?"
"Choshi wa doo?" would be a familiar way of asking about someone's physical condition.
Depending on what connotation you are going for, there would still be more ways to express other possible nuances of the question.