Documentation is the key and it's hard to come by. The Garand Collectors Association does some articles and spotlights on certain rifles from time to time and there is some cool history that can be verified in rare cases, but most Garands are just commodity items; in that their issuance, usage, and travels was not recorded in a way that can now be retrieved.
That's not to say that certain manufacturer models or serial numbers aren't prized, but by and large, there is no usage history that can be verified on +90% of them other than a mfg. date.
Unfortunately there are 10's of 1000's of those rifles in the hands of people who are convinced that their grandfather or great grandfather carried that exact Garand they now have, into battle. They fact of the matter is that the military didn't allow soldiers to get off a ship and carry their battle rifle back home with them. The vast and overwhelming majority were purchased by veterans after they'd already been home for years...... through mail order surplus catalogs. In the 50's and for years later, one could buy a Garand via mail order and 1000's were purchased that way.
Granddad bought one and said "This is just like the rifle I carried", to his family. Years pass and that quote has morphed into "This WAS the rifle my great grandfather carried."
Unsuspecting collectors get "taken" by historical claims that are dubious at best, and on the flip side, family members overvalue "their rifle" by attaching a historical tag to a Garand that may never have seen combat.
That in no way diminishes the history and collect-ability of any Garand, but it does need to be considered when buying one; especially one that has a story with it that is causing the asking price to be above market.