Ya, its an issue if the vehicle or VIN is unavailable.
You are correct about the pull the plug, try and feel the dish of the piston for the LQ4, flat tops for the LQ9.
If its a 6.0 from a 1500HD, 2500, 3500 its an LQ4. Sierra AWD C-3 and GMC Denali pickups use the same LQ4 but are rated at...