Add a perk progression bar, just like RPG's experience bar, showing your progress with your current perk. Could appear below, between your life/vest and ammo.
Tied to it, there could be a pop-up (like when you get an achievement) showing some specific points towards getting a new level with each perk (Confusing, I know :S). When you reach 25% of required whatever to gain a new level (for example, reaching 50 headshots to get lvl1 sharpshooter perk (I believe you must get 100)) the pop-up would appear.
Tied to it, there could be a pop-up (like when you get an achievement) showing some specific points towards getting a new level with each perk (Confusing, I know :S). When you reach 25% of required whatever to gain a new level (for example, reaching 50 headshots to get lvl1 sharpshooter perk (I believe you must get 100)) the pop-up would appear.