If there is a mint mark on the silver dollar, you can find it by looking at the side of the coin that has the eagle, beneath the word "one" and above the feather that almost reaches the edge of the silver dollar.
Think of the area beneath the word "one" and the feathers of the eagle directly beneath it as the top of a small upside down triangle. If present, the mint mark is in the bottom point of that triangle. It's important to be accurate, as the mint mark is one of the factors used when determining the value of the silver dollar.
Not all 1923 silver dollars have a mint mark. However, if it does, the letter is a very small S or D, so you may want to use a magnifying glass to identify it.