"If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck."
Or an alternative is the "two foot" rule Both of which can be applied to model trains.
I find both apply quite well to most rolling stock, especially stock that has some poetic license, is not super detailed, or has some technical compromises, paint shades a bit off, but visually looks close enough to what it is mimicking to pass the "duck test" or "two foot" rule. I do appreciate and respect that for some the "duck test" is a bridge too far.