These can be used interchangeably,
Where are you headed?
Where are you off to?
Where are you going?
The difference between past simple headed and present continuous heading is very slight. It's one of motion, but the two are interchangeable. For example, Max sees Sam walking by and asks, "Where are you heading?" because Sam is on the move, moving towards a destination. Max could also ask, "Where are you headed?". No difference in the meaning expressed.