Trees produced up to circa 1977 were made of a harder plastic and are a slightly less bright shade of green. In addition, the outer edges of the early trees are more angular than modern examples which have subtle rounding at the edges.
Beginning in 1978 a softer plastic was used-- the color of trees produced during this period is a more traditional green shade.
Sometime during the late 1990s the plastic used became less malleable, although not as hard as that of the original trees from the 1970s.
Note that these differences are quite subtle and not noticeable to many buyers and sellers. Sellers who would like to distinguish should make a note in their listing, and buyers seeking a particular version may wish to contact sellers before purchasing (although please be aware that the differences may not be readily distinguishable by some, particularly if one doesn't have the different versions side by side for comparison).