1942 w mercury head dime what is it worth?

That's not a mint mark. It's the monogram of the designer A.A. Weinmann. The West Point Mint did not open until the 1980s and only strikes commemorative coins and cents.

If your coin has a mint mark it will be on the back near the Word ONE. Blank = Phila, D = Denver, S = San Francisco. Regardless of mint mark it's worth about $1 for its silver content.