I spent some years at a GM dealer and on occasion had to verify VINs.
The frame will have stamping of the VIN, and may or may not be complete, it WILL have the Sequence (last six numbers) stamped.
The VIN plate will be on the Driver's-side on either the door jamb or the front corner of the dash. The exposed portion of the rivets may or may not be round or hex - if they are hex, the chances of authenticity are very high.
The engine pad will have the Sequence number on it as well as Alpha Numeric prefix.
The transmission, if original equipment will have the Sequence or last four of the Sequence on it - this also applies to the rear differential and not necessarily to the front if equipped.
There is also a Sequence number stamped under the cowling on most GM vehicles. You may not see it due to other welded panels obscuring the numbers.
Modern vehicles will have the entire Serial Number stamped all over the place - fenders, radiator support, cowls, floor pan, etc.. This as a result of gyppo body shops rebuilding vehicles without following proper repair procedures and of theft.
On your truck, aside from the Sequence Number, you can pretty much predict the first seven Alpha-Numeric Characters.