This is what a current USAF fighter pilot says:

"Some fighters can "see" transponders. I'd recommend Mode 3-1200, just
like other VFR traffic. However, we don't always go around
interrogating them, especially in a MOA where there is usually less
civilian traffic. The military radar controllers can definitely see
them--that is a great way to be seen and then they will point the
traffic out to the fighters, usually. Sometimes we work quite
autonomous and it might be of only marginal benefit, though.

"If I had to choose to fly in a MOA or not in one during "duty hours", I
wouldn't be in one as a civilian without talking to a controller to know
if it was "busy" at the time. Sometimes you might not have any choice,
I realize. Sailplanes are quite difficult to see in a fighter depending
upon aspect, and they don't always give a nice radar return, either."