The ABSCM checks/tests the system constantly. Other than that, you need a fancy computer. If a wheel speed sensor was flaky enough to feel it, you should have a code and the ABS lamp would be on. A simple code reader won't show the code (need the fancy computer, or a $$ scanner/reader).

Sounds like a warped/sticky rotor. You can feel that, and it won't throw any codes. If the ABS is actually actuating (hear/feel the ABS pump pumping), you're gonna need that $$ scanner, or some time in a shop that services ABS.