We can tell the soldier above was in the cavalry because of the gold which was hand painted onto the plate by the photographer.  Yellow was the branch color for cavalry in both Union and Confederate armies.  His role as a musician would be evident even without the bugle in his hand by the pattern of trim along the front of his jacket.  Musicians were not supposed to be targets in battle, so the trim on the front of their uniforms was designed to let everyone around them know that they should not be targeted.  This did not always work, however, as musicians were one of the primary ways the armies would communicate, and disabling the enemy's bugler was a way of preventing them from relaying orders.