If it is a "s" quarter it will always have the S mintmark. You can find this mintmark usually on the back of the coin. The exact location will depend on the type of quarter.
It would be located on the right of the quarter on the observe nest to the ponytail.
If you mean a 1966 American quarter, then none.
If there is no mintmark under the eagle on a quarter then the coin was minted at the Philadelphia Mint. That's the only mintmark position for Washington quarters dated 1964 and earlier. The mintmark was first put on the front of the coin in 1968.
Zero. A 1966 U.S. quarter is 0% silver.
Which one? A date and mintmark please:
The mintmark is a "D" not a "P" and it's just a quarter.
It would be located below the wreath on the back(tail) of the coin.
If you mean a mintmark? No US coins made in 1965, 1966, and 1967 have any mintmarks.
If it has a mintmark its on the reverse under the wreath, it can only be a D or S.
If you mean it has no Mintmark, value depends on the year it was struck.
No U.S. coins dated 1966 have a mint mark. The '66 quarter is worth 25 cents.