Army: The army originally had the drab olive color for clothing in 1938, they were designated with the 13 star button on their trousers. The herringbone cotton was stronger than regular twill cotton because of the broken weave and it was necessary to have such strong fabric during the war. When looking for Army clothing, always keep in mind the buttons, they must have the 13 stars, it makes searching for actual clothing or repros much easier.
Navy: You might recognize the wreath with the single star at the center, it's the symbol of the Navy, and one that was need to differentiate it from the Army because they used the same type of herringbone twill cloth. However, you'll see that the colors of the Navy have changed to blue, white, and gold instead of the drab olive color but the buttons are still the same no matter what color you decide to have.
Marine Corps: This one is fairly straight forward, the buttons were printed with their proper branch on it, and then others were created with the weaving/overlapping design. In any case, this is the easiest button to spot but once you know the difference between all the buttons, you'll realize that the articles of clothing you've been searching for are much easier to find. A little bit of knowledge goes a long way so go on and add military clothing to your wardrobe today.