From my re-search.
The stats are stored on Steam (And maybe on Treyarch Servers, for leaderboards)
+ When you edit your XP amount / Prestige.
Look in the barracks.
Do you see?
(The orange XP bar is not correct with your current XP)
So from this point, it's easy to detect you:
1: XP don't match with Steam Stored Data
2: XP don't match with Leaderboard Score
3: XP don't match with the barrack XP