There are many subtle difference between the two years in addition to what's been mentioned:

- tail lights are slightly different between 72 and 73-74
- grill inserts are different between 72 73-74
- fender radius is slightly smaller on 72 than 73-74
- inner fenders are drastically different from 72 than 73+ due to the suspension overhaul Chargers got that year
- if the car still has bumperettes, the 73-74 will have much larger (and uglier -- usually taken off) bumperettes than the 71-72
- firewall on 73+ will have more emissions "stuff" on it than 72 such as OSAC valve
- 72 had hideaway headlights as an option - 73+ did NOT

This is by far not an all-inclusive list but covers most of what you could easily spot to know if its a 72 or 73 Charger. Keep in mind some of the parts can be swapped between the two. For example the 72 front end can be swapped on to a 73/74 to give it the hideaway headlights.

As said before, if the car as the triple "opera" windows, its a 73/74. I am pretty much certain that no others years had them.

If you are still not sure, post a pic of some of the parts I mentioned above and I am sure somebody will chime in and tell you for certain.