Follow the steel brake lines out of the master cylinder to see what they go into. If there's a brass block under it, on the frame rail, with three additional lines going out, that's the standard combination valve, and you don't have anti-lock brakes. If one steel line goes to a larger valve assembly that has one more line going to the rear brakes, that's rear-wheel-ABS. You'll find a speed sensor with two wires on top of the rear differential. If you have four-wheel ABS, the hydraulic controller near the master cylinder will be a lot more substantial. There will be speed sensors on each front wheel. You'll see the wires going to them clipped to the flexible rubber brake hoses.
There will be a yellow ABS warning light on the dash that lights up for six seconds during the system self test when you turn on the ignition switch. Also, most trucks have a sticker in the glove box listing the optional equipment. That will list which anti-lock system you have.