The steps for checking your signal lights include the following

Step 6: Check the Signal Lights
Get in and Turn off the Lights
• Turn off all the lights.
• Turn on the stop lights (apply the trailer hand brake or have a helper press the brake pedal).
• Turn on the left turn signal lights.
Get out and Check the Lights
• Left front turn signal light is clean, operating, and the proper color (amber or white on signals facing the front).
• Left rear turn signal light and both stop lights are clean, operating, and the proper color (red, yellow, or amber).
Note: Checks of the brake, turn signal, and 4-way flasher functions must be done separately.
[Commercial Driver's License Manual]