The only really reliable dating i know of is postmarks. I will pay more for a postally-used card with a clear date on it; ironically, they often sell for less because they won't slab better than a 1.5.
Mailed in 1957 from Brooklyn? Yeah, that's a rookie. And this is from Koufax's final season:
The most you can do otherwise is rule out certain date claims as anachronisms (e.g., a Los Angeles Dodgers hat cannot be made pre-1958). Besides that, dating is inductive reasoning: a certain cap or uni style only proves the earliest date of manufacture, not the actual or last.
I have seen this card sold as a Gibson rookie or with any of a variety of dates attributed to it. This
photo has to be 1962-63 because the Cardinals first put front numbers on the jersey in 1962 and went to red caps in 1964 for home, all games in 1965. A claim that it is pre-1962 is an anachronism: cannot be true. 1962-63 cannot be established either. The
card could have been made any time after the photo was taken. The absence of a ZIP code on the back only proves that the back was typeset before McCarthy created a ZIP code back for this card, not when it was printed, though it does suggest that it was printed pre-ZIP code because McCarthy did put them on the cards eventually.