Many supersonic fighters have a stabilator to control pitch. My question arises when I cannot see any trim-tab on the stabilisator (e.g. on the F-16 or on the F-22). Other means to trim an aircraft without trim-tab and without using the stabiliser as a trim must exist. What are those means?
Trim is simply a force that offsets the neutral position of the control surface. On aircraft with mechanical controls it is provided aerodynamically via trim tab, but it can also be provided by springs, hydraulics, or simply by adding the selected offset in the fly-by-wire computer. Since F-16, F-18 and F-22 are all fly-by-wire, the last option is most likely.
In fact, all these fighters are aerodynamically unstable, which requires using “cooked” controls where centred stick commands the computer to maintain straight flight and the computer then adjusts the stabilators accordingly, doing small corrections all the time since the flight path would quickly diverge without that. So there is no trim in the traditional sense at all.
Pitch trim on an aircraft with a stabilator is normally accomplished by moving the entire stabilator and holding it in its new trimmed position either mechanically (using the same mechanism as normal pitch control), aerodynamically (using an "anti-servo tab"), or some combination of the two.
The concept is similar to stabilizer trim as discussed in the other question, only instead of moving the "fixed" forward part of the horizontal stabilizer the entire control surface is moved.
The Pilot's Handbook of Aeronautical Knowledge has a good illustration of an anti-servo tab: