* Check your owner's manual: This is the most reliable source. It will specify the lug nut size and thread pitch.
* Look at the lug nuts themselves: The size is often stamped on the nut itself.
* Check the wheel: Some wheels have the lug nut size information stamped on them.
* Use a lug nut gauge: Auto parts stores sell these tools to quickly determine the lug nut size.
Without knowing the specific year and model of your 2500 Chevrolet pickup, I cannot provide the correct lug nut size.