pHEnomIC

I noticed today that i could lock my brakes up at high speeds. I never tried it before so i decided to on open highway. At 50mph, i just slammed the brakes and instead of slowing down like ABS does, it just locked up and kept sliding. I tested it a few more times and same thing happens (Low trac light comes on when i do this). I am now wondering if my abs is broken or if i don't have it at all. How can i tell if i have it for sure? I ended up driving into a ditch a month or two ago during a rainstorm and i think the fact that i slammed the brake instead of pumping it had something to do with it.

I have never had an abs inop light but i do have ABS fuses in my fuse box all of which seem to be working. Is there any way i can tell if i have abs or not and if i do have it, what do you think is causing it to malfunction? I have new pads and i think my rotors are ok. I plan to flush the system some time this week.
97 z28 btw.

5.0THIS

You can see the ABS block/controller underneath the hood. And there will be ABS sensors on the front hubs