First of, check out the sticky thread in Stock Trucks called LUV year by year changes. I have not updated it in a while and it could be refined down some, but that should give you some ideas to go on. Find it here:
http://forums.luvtruck.com/viewtopic.php?t=2083
I'll give you a brief run down here though.
74, narrow front bumper, tail lights under the tailgate, early style motor with aluminum valve cover
75 same as above, but tail lights up beside the tail gate, tail lights have red turn signal lenses.
76 wide front bumper, new style motor(black steel valve cover), tail lights same as 75
77 same as 76 but tail lights have amber turn signal lenses, usually the bed will have three tie downs on each side and two on the tail gate, 76 and earlier trucks had 4 on the sides and 3 on the gate
If you still have a VIN tag on the cab you can also check the 6th digit to get the series number. So if your VIN starts out like CLN14582... it is a series 5 truck, which is a 1976 model year. (Series = year +1) They skipped series 7 so 78 and up the series matched the year, and A means series 10. 81-82 trucks have a longer VIN, digit 9 is the year (A=1, B=2).