A relatively easy way to keep instructors & checkers on their toes would be a confidential rating system. The bottom percentile scoring instructors and checkers can be either released back as regular line pilots or given the opportunity to improve. This allows a regular removal of bad apples and influx of new talent to the training department.
The key would be a properly confidential rating system that cannot be abused by disgruntled checkees/trainees.
But I suspect the old boys club would be aghast with such a thing in most ‘modern’ airlines.