If a floating line drifts to either vcc or ground the logic will read it as such . Thus Neither pull ups or pull downs will result in a float definition. I am assuming that only one of the sides (external or internal) is floated when the test is made.(If all connections are floated it is untestable and does not matter) To guarantee float you should bias the pin to near mid point and insure that when not in float the circuit drives to the correct limits. To detect it look for the oscillation that will probably result. I guess the real question is why you need to know this? Your program controls the port state and your overall design should control the extertal circuits state Tery