I bought a M1 Garand at the CMP in Alabama. This thing looks brand spanking new (wood is new from CMP). It is a Springfield Armory model from 1955. How can I tell 100% if this gun has been fired?
Well, you would have to look for traces of finish wear, copper fouling, and powder fouling.
It used to be that all CMP rifles were test fired before shipment.
The Greek rifle shipment had lots and lots of essentially unfired late model Garands. Yours is likely from that category, but the wood was damaged or the metal finish is worn in places, so the CMP did not sell it as correct grade. The correct grade rifles I handled were in outstanding condition. If they had ever been shot, it was during factory acceptance. They used to sell Collector Grade rifles which were virtually new and guaranteed to be all correct from the factory.
The CMP has always had the problem of having lots of new rifles with damaged wood and damaged metal. Original stocks were always at a premium. If you go down to the CMP you will see a smattering of rack grade rifles that the barrel and receiver are correct, look unused, some surface finish wear due to shipment, but a couple of parts are mismatched. You will find combinations of switched trigger guards, operating rods, bolts, and stocks. A minimum of two marked parts will be incorrect. These were all correct rifles which had stock damage, or finish damage that kept the rifle out of the “Correct Grade” category. To stop complaints from people who paid $1300 for Collector Grade, or $1000 for Correct Grade rifles, and then saw their neighbors all correct 90% $500 rack grade Garand (and that happened), the CMP switches parts to ensure that nothing below Collector or Correct Grade goes out the door with all matching parts.
If you attempt to swap parts, to make the rack grade correct, the CMP has posted a sign that informs you that if they catch you doing this, they will kick you out of the store and won't let you back in.
However, if you go down there with a bud, and between the two of you, buy enough rifles, you can swap parts with your bud and create all matching rifles. I have done this.
Your rifle, with replacement wood, may be all original and all correct with respect to its metal parts. Because the wood has been replaced, it is not "all matching".