How can you tell if you're looking at an older M1A with military parts?
There are many ways to tell. Do you want the long version or the short version?
In the interest of time and space I'll give you the short version.
Basically USGI builders that were under contract to build the M14 were as follows:
Harrington and Richardson- Most common maker of complete M14 rifles. H&R parts are stamped "HRA".
Winchester- Maker of complete M14 rifles. Winchester parts are stamped 66118.
TRW (Thompson Ramo Woolridge) - Maker of complete M14 rifles. Parts are stamped "TRW"
Springfield Armory- Parts marked "SA" on an M1A may or may not be original GI. The original Springfield Armory is located in Springfield, Massachusetts and was closed by the U.S. government in 1968. The current "Springfield Inc." of Illinois just took over the non-trademarked name of a closed U.S. government arsenal. SA Inc. does a great job of misleading people to think they have some connection to the original U.S. arsenal, using marketing slogans such as "The Oldest Name in American Firearms". Nothing against SA Inc., they helped revive a classic rifle, produce a receiver legal for the average citizen to own, and have done a good job of it. It's not their fault that the supply of original GI parts ran out. The M1A is an expensive rifle to make and SA Inc. had to start making their own reproduction parts or else close the doors. Some of their repro parts are ok, some aren't so good and it shows.
Identifying original Springfield Armory parts of government manufacture can be a bit tougher, since the current commercial venture marks their parts similarly. For example, original (government) SA op rods are marked with part number 7267064 SA all in one line on the op rod. "SA Inc." reproduction op rods might have a -2 after the 7267064 part number with the "SA" stamp being above the part number instead of in line with the part number as would be found on an original GI op rod. I could go on and on, but the bottom line is buyer beware of SA marked parts. They may or may not be GI so do your homework.
SAK- Did not make complete rifles but did make op rods and barrels under government contract. My memory is failing me right now. I forget what "SAK" stands for, but I know they were definitely a government contractor.
This is just a quick and dirty rundown of original M14 manufacturers. You can learn a lot with a quick disassembly of a rifle into its basic components of trigger group, barreled action, and stock. My advice is to check critical components like op rods, trigger groups, hammers, sight assemblies, and to a lesser extent barrels for original GI stampings. Typically GI flash suppressors have splines that are more squared at the ends versus commercial versions that are rounded. There are many rear sight variations from GI contractors that I won't even begin to get into.
What does all this mean? Maybe nothing, or maybe everything. It just depends on whether you value having the best of the best in terms of parts on your rifle. Most of SA Inc's repro parts are good enough to get the job done, but some aren't so good, extractors being the number one problem. Go armed with as much information as you care to and spend your money accordingly.