Compare the height of the sleeve cap to the height of the armscye.
The sleeve cap should be at least 1/2 inch longer than the armscye. This allows for the shoulder curve. The sleeve must be eased in the cap area to allow for the shoulder curve. Otherwise, the sleeve would fall straight down vertically and pull.
If there is one inch or more extra in height, the sleeve cap will need to be gathered. Gathering will cause a rise in the sleeve at the seam.