This article is from the Toyota RAV4 FAQ, by Ralph Becker with numerous contributions by others.
You should get feedback in the form of an audible "chunk chunk chunk"
sound AND a strong pulsing of the brake pedal. This is normal and
you should NOT release the pressure on the brake pedal or attempt
to modulate the brake pedal pressure. If you live in the snow belt,
a good way to test your ABS is to find an open, snow-covered area,
drive to about 30MPH, and slam on the brakes as hard as you can.
The described effect should be obvious.
Warning: Don't test or show off your ABS system until after the
break-in period, because it can create hot spots that can cause
excessive brake system wear.