Honestly, IMO, the answer is: fellow collectors, such as the ones on this board, coupled with one's own research, and then making one's own, final determination for one's own collection. For some players, especially Post War, it is often an easy call. For others, not so much, and there is no one true and right answer. If you go by the PSA Registry, for example, they'll tell you that Babe Ruth's RC is his 1933 Goudey. Ask anyone else, they'll probably say the M101-4/5. Some others might say his Balt News. I know a ton of guys that embrace Gehrig's 1925 black/white tint Exhibit as his RC. I believe the PSA Registry anoints his US Caramel his rookie. For DiMaggio, I consider his rookie to be his Zeenut Batting, but he's not in pinstripes there. I'd definitely next look toward his V355 from 1936, in which he's in a Yankee cap. Some will tell you his Goudey is his RC (for some reason I am not a fan of that cartoonish card). It's an area rife with debate.
Maybe I just have a problem with authority; I don't like TPGs telling me what grade my cards are and I don't like an external authority telling me that Gehrig's RC is his 1932 US Caramel
