Weigh the player and measure his chest size and shoulder width. These three measurements help determine what size shoulder pads a player should wear. Shoulder pad sizes are pretty universal and manufacturers usually should have sizing charts available.
Place shoulder pads on the player and check to make sure his collarbone and shoulder joint is covered by the top of the pads. The pads usually have an arch at the top that is indented to cover the collarbone.
Look at the outside edge of a player's shoulder; make sure there is padding covering this area. This is a crucial area because a lot of contact is made on this part of the pad. If the pad is not covering the player's shoulders, immediately rule the pads out for that player to wear.
Grab the pads and shake them as the player wears them. This will tell you if the pads were fastened tight enough. If you can easily twist the pads on the player, the straps need to be tightened.
Have the player put on his helmet and jersey to make sure everything fits properly. This will help you determine if the jersey is the the proper size.