If unsure if a trailer has an Anti-Lock Braking System (ABS), you can:

Question

If you do not know whether or not a trailer is equipped with ABS, look under the vehicle for the electronic control unit and wheel speed sensor wires coming from the back of the brakes.

Accepted Answer

Look under the trailer for wheel speed sensors coming from the back of the brakes.

Answer

Remove a tire and look for the ABS sensor on the axle.

Answer

Assume it does have ABS.