Don't forget the grills.
There are two styles: "chicken wire" and "corrugated".
All SD40s and the early SD40-2s had the chicken wire ones (a wire mesh), while only SD40-2s had the corrugated ones (closely spaced vertical slats).
Also, the trucks are a good bet. Look for the "horn" at the end toward the fuel tank. The only time those aren't reliable are on former Conrail units, which all had Flexicoils (like SD40s).
There's also the cab overhang too: there's a slight overhang on the cabs toward the long hood: the top plate extends past the wall.
But the dead giveaway is the sight glass on the engineer's side of the long hood at "head height" back by the radiator grills. If there's one present, it's a -2. If not, it's an SD40 (or made by Life-like/Walthers)

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