what specific aspects of the design ethic would clue you in to the notion that it was an older design, rather than something that was designed as recently as last week?
Cylinders butted up against rectangles butted up against disks with no transitions; exterior lamps fastened to the hull, a great big round dish antenna jutting out the front - the appearance of the TOS ship compares to, say, the TMP or particularly the TNG ship in the same way that a 1954 Chevy sedan compares to a 1996 Ford Taurus. You can argue about which design is "better" or more "classic" but it's not at all difficult to assign a chronology to them based simply on the sophistication of manufacture and finish.
Or, if you prefer:
Now, you know what? I'd rather own the older of those two Mustangs, but that's just me - there's still not any doubt which one dates from the mid-1960s and which is contemporary. You can tell by looking, and you could tell from your general knowledge of cars and design even if you'd never seen a Mustang.