Maybe the amount of the zap, next to the zap sign on the pfp. Or just change the colour slightly of the zap sign to indicate that it was zapped? Not totally sure how that works long term though…

Or…when you long press the zap button, you see the ln adress and under it you could see a history of the last n zaps I made to that npub ? Maybe this option, or something similar, is less intrusive