If you aren't seeing it happen I don't think there is much you can do. You could try putting food along the fence line and correcting the dog firmly if they go for the snack...but that doesn't mean that they'll avoid temptation if you aren't there.
Most likely you'll have to be diligent in catching your neighbors at tossing goodies and give them the "really?!? I told you my dogs can't eat that stuff" speech again, and again until they get the idea.
Patton CGC BH
Chief fetch fanatic
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