Looks like you got the car from Bucks County Mustangs. Are you from the Philadelphia area? I bought my 68 coupe from there, watch out with that guy he can be shady, and his prices are high. You can talk him down though, I talked him down 2K on my coupe.
Easy - like 65-66, it's all in the grille. 65 comes with bar across the grille while 66 doesn't. It's the same way with 67-68.... 67 has the bar and 68 doesn't.
Looks like you got the car from Bucks County Mustangs. Are you from the Philadelphia area? I bought my 68 coupe from there, watch out with that guy he can be shady, and his prices are high. You can talk him down though, I talked him down 2K on my coupe.
Actually even if you are the original owner of your car and it has the original antenna, you can still not use this as a means for comparison. Ask any original owner who has lost points in a concors showing and he will tell you that antennas were often installed at the dealers, and that they got in the habits of grabbing the antenna's out of the trunks upon delivery and throwing them all into a pile, then just grab the next one and install it. My car is actually a victim of this. It had the correct square base, but instead of a round shaft it has a tear dropped one. Original yes, correct no.
The easiest identifier aside from the marker lights on the fender is of course the quarter panel scoops. 67' and 68' are night and day.