There are two ways to be able to differentiate the years between the 1979-1982 LTD 'S'. First of all, the 1979-1980 had a slotted front bumper like this one:
http://www.imcdb.org/images/046/448.jpg

the 1981-1982 LTD's had a solid front bumper like this one:
http://www.imcdb.org/images/026/061.jpg

Also, the 1982 was the first and only year the LTD 'S' had the blue ford oval on the grill:
http://www.imcdb.org/images/108/969.jpg