Familiarity (NYC Hudsons are my favorite loco, only challenged by the NYC Niagara, occasionally). Price. Era (except for the 700E and its decendants - 773, 785, 783, re-issued 700E - all PW Hudsons are small, with either no prototype or a Santa fe prototype: 2055, etc.)
All Lionel Dreyfuss and ESE Hudsons are scale - but note the little PW 221 Dreyfuss "Hudson" (it's a 2-6-4; cute as a button). Anything listed as "J3" or "J3a" is 1:48 - except the Lionmaster J3a. The Commodore Vanderbilt is 1:48; the 1990's 5344 J1e with the incorrect Vanderbilt (ironically) tender is 1:48.
Is there a book? I'd like one, too.
Lionel website - look up Hudson, including archived items.
Looks - pretty much, if it isn't 1:48, it will look it - the Lionmaster J3a being an exception, pretty much.
Price.
Curve requirements - if it's under 054, it's sub-1:48, pretty dependably.