The answer to the question of how can I tell if my M1A has GI parts...
markings of SA (Springfield Armory) , 66118 (Winchester), TRW, HR on operating rods,
barrels, bolts and trigger groups are all indicators that you have GI parts.
The USGI Springfield oprod will have 7267062 SA on it , on the SA Inc oprod SA is on line separate
from the number
The Gas Cylinders are harder to tell, their are pictures on the web somewhere that show the difference between cast gas cylinder and GI forged, but I dont have them bookmarked.
The GI parts that you want to have in my opinion are
Trigger Group
Bolt
Operating Rod
Gas Cylinder
Rear Sight assembly
Front Sight (if you are going for a service rifle clone)
The barrel can be GI or one of the Smith enterprises barrels
For your operating rod spring guide and gas piston , get Sadlak
If you are doing a crazy horse build get a Smith gas lock front sight and Vortex
Stock, get one of Fred's cheap fiberglass stocks and spray paint it.
And you might as well get the other small parts as GI surplus as they can all be had for reasonable.
Link to info on identifying USGI boltsCheck some of the other forums out there, when I built up my M1A with all Winchester I was able to scour the internet and find alot of info on parts ID for the M14 out there.
There is actually a book online somewhere.... has everything in it....