
How to counter:
1) Reconnaissance-in-force. You can send a small fleet to the high orbit that will show if there are any Scourges.
2) Sensor Phalanx can show you the full composition of any fleet at high orbit.
3) With a hired Telepathist from a hidden satellite, send the fleet into high orbit and in the Overview menu look at "See Object", if the object visibility has become available to other players, then there are scourges in high orbit.