The two most obvious things when you're first looking at the harness are the location of the fuse panel and the presence (or not) of the two rectangular firewall connectors.
From '66 to '70 they used the firewall connectors and the fuse panel was on the floor/firewall by the driver's right foot. In '71 they got rid of the two main pass-through connectors in favor of pass-through wires with grommets. At the same time the fuse panel moved to the glovebox.
For a more narrow range, it might be hard to nail it down. A '66 harness could be different in many ways from a '68, and a '77 would be different from a '71. Mostly in the little details, some of which are at the ignition switch.
For a '73 to '77 harness, there is a White w/black stripe "stator" wire that runs from the alternator to the choke on the carburetor.
Sometime in '73, but for sure in '74 to '77 the use of under hood connectors went from the old style oval shaped rubber connectors, to the later round-ish hard plastic "Dura Spark" style connectors.
For '74-ish (for sure '75 though) to '77 there are additional wires for the electronic ignitions. Also there might be two wires on the center post of the ignition switch rather than just the single Black w/green wire of the earlier models.
Location of the voltage regulator under the hood changed at least twice, but it's not quite as good an indicator as some of the other stuff.
Good luck.
Paul