According to the FlexAPI documentation, the Radio class has a read-only property IsGpsdoPresent. My 6700 has the GPSDO installed, but the IsGpsdoPresent property shows false. I poked around in Radio.cs, but I don't see where the property is set.
It is set in Radio.cs file in the ParseOscillatorStatus method. Search for _isGpsdoPresent You must be connected to radio for this property to be set. Also note that until a GUI client connects some radio object properties are not update / set.