Assuming one can "see" the calling and working stations, watching to see if the DX is TX down around 300 - 500 Hz, the calling stations above 1000 Hz (except for those not knowing how to use F/H) then if the calling stations suddenly move their TX to or near the DX TX frequency to complete the exchange then one can assume the DX is using F/H. I have not seen any other objective way to determine if or if not F/H is in use.
