Generally speaking, the DRL's are turned on when the truck is in gear, the outside light level is not low enough to activate the headlights and the parking brake is not engaged.
I have tested mine while inside my garage by placing a lit flashlight on the dash pointed at the light sensor, blocking the wheels, leaving the engine off but turn the key to run position and placing the shifter into Drive.
In that configuration, the DRL's should light if the bulbs and or sockets are good.
Also, while you are out driving in the daylight, watch for the reflection of the front of your truck in glass storefronts or the vehicle in front of you at a stop light.
Odds are, if you can't catch the DRL lights lit up when they should be, one or more of the bulbs and or bulb sockets are burned out.
It is not uncommon for the sockets to burn to a crisp from having run for extended periods of time over the years.
I've had to replace sockets on all of my vehicles with DRL's over the years.