The Brunette on the left is Vintage, and the Black haired boy on the right is 40th. The 40th figures generally have a paler complexion than the one represented here. Note that he is shorter than vintage, and sports a different face, but we'll get into that later.
The vintage figure (left) has a copyright marking that reads:
G.I. JOE TM
COPYRIGHT 1964
BY HASBRO R
PATENT PENDING
MADE IN U.S.A.
There are a few different version, go
here to see them.
The 40th copyright marking reads:
G.I. JOE TM
C 2003 HASBRO INC.
ALL RIGHTS RESERVED
MADE IN CHINA
The 40th figure (right) has a sculpt that is rumored to be a composite of several vintage sculpts. The result is something that never looked quite right to me (except on a redhead).
The Connector pins on a Vintage figure (left) are milky white. The same on 40th figures (right) are dead clear. Added note, in my experience the connector pins on 40th figures are very fragile.