Here you are at the airport and a DC-8 taxis out. Is it a -71 or a -73? mmmmmmmmmm? There are a lot of internal structural differences that can't be seen; the -73 has 6' more wing, you'd need good eyes to see that. Look at the engine nose cowls (at about the 1:30 and 10:30 positions)(Gee, I hope you don't have a digital watch!!
), if there are a pair of strakes (called rams horns) on the nose cowl of the engines similar to the DC-10/MD-11's, it is a DC-8-73 (-72's also, but the fuselage length is easier to see). If they are not there, it is a DC-8-71.