The short answer to your question is no. There is no practical way to determine if any sight has been on a rifle was put there during manufacture. The sights were made as separate parts and maded & finished in batches. Even parts made at the same time may have different colored finishes if they were finished indifferent tanks, in different solutions on different days before they were assembled.

Scott Duff's book on WWII Garands has a lot of info on lock bar sights the so-called flush nut or type I, type II, type III, etc. The info he presents matches the s/n of rifles to the month they were produced and to a variety of parts, inclusing sights, that would be expected on a rifle produced in any particular month.

Most Garands were reworked after the war & fitted with the T105E1 sights & modified op rods. Also, many Garands were broken down and reassembled at unit & depot level during & after the was. There was never any concern about returning a particular part to a particular receiver. In that sense, practically every garand is a mixmaster. Still, many Garands show up with the LBs. At the CMPicon, perhaps 2% of the Garands came with LB sights. Since most of these were Greek & Danishicon returns, I would consider these "original" to the rifle, particularly if they were the right type for the s/n. The CMP usually replaced only parts that were worn out or damaged, the probability is very high that these sights had been on the rifle since WWII.

The whole concept of an "original" Garandicon is a will o' the wisp and descends into philosophy. Unless one can certify that a particular rifle is exactly as it left SA as new production, is it original in the sense that it has all the parts it was originally made with? Will just one part changed out later makes in "not original"? If a rifle had a damaged/worn/obsolete sight or swivel or bolt that was replaced by a GI armorer with one of later vinatge, isn't it still "original" in the sense it meets the Army's definition of a Garand? Is a rifle less "original" if one GI part is replaced with another GI part by a civilian? I know many collectors want "original" rifles & do so by swapping parts. Personally, I consider any Garand used & maintained by the US or it allies as "original" but that rifles with import marks on them as "not original".