Well, it depends on how old the system is. Older systems require jumping the compressor out to force it to run just to check it / add refrigerant, for example.
Your best bet is to have the truck running, set it to MAX AC and fan speed all the way up, and look at the compressor. If just the belt and outer edge of the pulley is turning, it is not kicking on. If the whole thing is turning, it is turning on.
I kinda need to know what year and make the truck is... >