Are there mint marks on all silver dollars?

No. Silver dollars minted at Philadelphia did not carry mint marks.

Mint mark positions are:

Seated Liberty dollars - under the eagle

Morgan dollars - above the DO in DOLLAR

Peace dollars - between the Word ONE and the eagle's tail feathers.

Mint mark letters are:

blank = Philadelphia

D = Denver

S = San Francisco

O = New Orleans

CC = Carson City

Dollars weren't minted every year at every mint, so there are many gaps in the series.