You could ask someone to press on the brake pedal with the engine off and the ignition key on. If you stand next to the trailer axles and the brakes work you will hear a clicking noise when the pedal is depressed. You could even raise one trailer wheel off the ground and feel whether the brakes are engaging. Make sure to raise a wheel from the axle that has brakes, as some small trailers come with only one braking axle.
When you jack up the trailer, follow the manual. Normally the jack goes under the frame near the axle but not under the axle itself as it's made of light tubing and will bend.
