What this coin looks like (obverse, reverse, mint mark location, special features, etc.): One major variety of the 1964 Kennedy Half Dollar featured accented hair on the portrait of John F. Kennedy on the obverse of the coin. This is believed to be the first design used on Kennedy Half Dollars and only 1%-3% of 1964 proof coins are of this variety (between 40000 and 120000 coins minted).
The widow of JFK - Jackie Kennedy - was unhappy with this design and wanted the hair in the area just above the ear to be less prominent and so the Mint revised the design to the "normal variety" design used to produce the remainder of the 1964 Kennedy half dollars.
There are a few ways to identify this coin. The "Accented Hair" variety has longer and thicker strands of hair in an area just above the ear (more accented). Also of interest on the obverse is the letter "I" in "LIBERTY". On the accented hair variety: The letter "I" is missing a serif on the bottom-left side whereas the normal hair variety has all 4 serifs.
There are things to look for on the reverse of the coin as well. There are 4 stars within the rays behind the eagle's head. On the accent hair variety - there are broken and weaker rays where the stars intersect the lines. Although on the normal hair variety the rays are solid lines. The last major feature to look for on the reverse is Frank Gasparro's "FG" initials under the right-side leg of the eagle. The accented hair variety has a "G" with a straight line and no serifs on it. Although on the normal hair variety: There is a flared "G" with a small horizontal serif on the "G".
All of these features are depicted by the arrows and captions in the example comparison image below. Click image to zoom in and view the full size presentation experience:
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD