To confirm that a trailer has ABS, you can:

Question

If you are unsure if a trailer is equipped with an Anti-Lock Braking System (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

Look under the hood for the ABS sticker.

Answer

Look at the tires.

Answer

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