score:14
There's no automated checking. This would require a database of license plates storing whether the requirements for the sticker are fulfilled, and this doesn't exist.
Umweltzone violations are mostly detected and enforced by officials patrolling parked cars. If you were caught that way, you'd usually find a piece of paper under your wipers, telling you that an official fine is on it's way (via paper mail). Sometimes the information on how to pay is directly added, sometimes not. It is possible that the fine is still on its way even without a paper saying so on your car, but that's already less likely.
As you were never parked and only driving, it is extremely likely that you just got away with it. The only way I could imagine that this were detected is if you were also photographed by a speed trap and that photo is of sufficient quality to prove that there was no sticker present. But you would have noticed that flash, then.