Detection 1: Constantly run HFDF on all GPS frequencies. You know where your own transmitters live. Process of elimination. Detection 2: Compare GPS velocity v

Detection 1: Constantly run HFDF on all GPS frequencies. You know where your own transmitters live. Process of elimination. Detection 2: Compare GPS velocity vector to INS velocity vector. For fixed installations, have two receivers spaced some distance apart and compare GPS location. Detection 3: Measure the time delay between GPS and your RTC. With a GPS disciplined RTC, drift should be low enough to make this viable. Detection 4: Look for jumps in GPS location or time. Mitigation 1: Send black vans, soldiers, or missiles as appropriate to rogue transmission locations determined by HFDF or the encoded position of spoofed signal. Mitigation 2: Frequency-agile and mobile GPS substitutes almost certainly have a prominent role in EW strategy. Of course, as you note, the details have to be classified, but it's pretty easy to speculate what form the overall strategy will take.