The year will also be cast into the top. A '68 4v head will have both "68" and "4V" cast into the top. The other, even better way is to look at the casting number on the bottom of the head. The 1st 2 digits will indcate the year. A '68 4v head will have a C8OE casting. Not all C8OE heads, though, are the 4v variety.
BTW, the first leter indicates the decade C=1960, D=1970, E=1980