Identifying which player is which on game day requires looking at alignment, jersey numbers, and pre-snap duties:
On a soccer pitch, locate the outer boundaries of the back four. When the opposition builds down the wing, watch the defender nearest the touchline. If that player presses out to meet the attacker while the two central defenders stay anchored in front of goal, you are watching a full-back at work. If your team has possession and a defender steps up alongside the defensive midfielder instead of sprinting down the sideline, you are witnessing the inverted full-back role in action.
On an American football gridiron, check the offensive backfield before the snap. If two backs align behind the quarterback under center, look at the front man. That is the fullback. He will almost always be thicker and broader than the tailback behind him. Watch his eyes right before the snap: he is not reading deep downfield safeties. He is staring directly through the chest of the middle linebacker, preparing to detonate the running lane the millisecond the ball is snapped.
