How can you check that the spring brakes will come on automatically on a semitruck?

Put the key in the "ON" position with the engine off, have the vehicle in gear, push in the brake control valve (and trailer supply valve, if hooked to a trailer). Hold the brake pedal down for one minute... a single vehicle shouldn't lose more than three psi/minute, and a combination shouldn't lose more than four psi/minute.

After that, you pump the brake repeatedly. You should see some sort of low pressure warning when the system gets below 60 psi (which is the hold off pressure for the spring brakes), and you keep pumping the brake. Then the brake control valve (and trailer supply valve, if applicable) will pop out, and that engages the spring brakes.