There were 11 players who appeared in both the 1967 and 1968 all star sets, including Willie Mays who also was featured in the SF Giants set in addition to the two "regular" ones. Willie in fact has more appearances in vintage Topps "oddball" sets than any other player of the time, but I digress. Other than those 11, (and Juan Marichal -- a yellow background in the SF Giants set and who was not a '68 subject) everybody else appeared in only one of the three sets, making the determination of the year of issue for the majority of subjects fairly easy. As it turns out, there are enough differences between the 1967 and '68 common subjects to allow a further parsing of who and when.
First, let's look at this 1968 uncut sheet of 28 1968 Discs offered by Huggins & Scott some time ago-a thing of beauty is is:
Here is the rundown of differences, showing the 1967 disc for each against the '68 sheet above for comparison. Some are hard to differentiate while others are quite easy, as you will see below.
Hank Aaron 1967-No Cap; 1968-Cap:
Orlando Cepeda 1967-No Cap; 1968-Cap:
Roberto Clemente 1967-A small bit of white shows through the "O" in "Bob" and the middle and bottom bars of the last "E" in his name do not extend into the blue background, leaving a small bit of white uniform showing; 1968-No white shows in the "O" and the middle and bottom bars of the last "E" touch the extreme edge of his uniform, leaving no white before the blue background picks up. I will show a '68 (Howard Chasser's I believe) below the '67 as the difference is slight. Ignore the color differences, those are just scan-related I'd say:
Frank Howard 1967-last letter of last name "D" extends into the red background; 1968-"D" is on the uniform shoulder:
Mickey Mantle: 1967-OF; 1968-1B:
Willie Mays 1967: San Fran. Giants set (shown): bust looks straight at you on yellow background, 1967 Regular Looks Left "F" in OF is on uniform shoulder with blue background; 1968 Looks Left (same pose as '67), "F" is into blue background:
Brooks Robinson 1967-faces left; 1968-faces right:
Frank Robinson "1967-the"OF" "O" is all on the white uniform shoulder ; 1968-half of the "O" is in the red background
Ron Santo 1967-"C" in Cubs is touching his hat; 1968-"C" is clear of the hat:
Joe Torre 1967-"O" in "Joe" intersects with uniform trim; 1968-"O" is above trim
Carl Yastrzemski 1967- the "R" in "Red Sox" bleeds into the hat; 1968-the"R" is clear of the hat with red showing:
The Clemente, Frank Robinson, Santo and Torre are particularly hard to tell apart. Howard and especially Yaz are a little easier.
As for 1967, we know the Giants set is from that year (or at least the proof run) as it shares shiny space with the regular set: