If unsure if a trailer is equipped with ABS, you can:

Question

Trailers and converter dollies built on or after March 1, 1998 are required to have Anti-Lock Braking Systems (ABS). This will be indicated by the presence of a yellow malfunction lamp. You can determine if older trailers have ABS by checking under the trailers for ECU and wheel speed sensor wires coming from the back of the brakes.

Accepted Answer

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

Answer

Try driving on a slippery surface and see if ABS kicks in.

Answer

Assume it does have ABS.

Answer

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