We provide internal training on this topic, and customize it to hit the problem areas as well as general best practices.  We also reference a book that ICMA put out, Effective Supervisory Practices Fifth Edition edited by Michelle Poche' Flaherty, for this and other leadership/supervisory trainings. 

I'm inquiring about whether your entity offers manager/supervisor training for those that conducts annual employee evaluations. If you use an external vendor to provide this training, could you share the details? I'm trying to ensure consistency in evaluations as it is crucial for legal reasons, and I want to address potential discrepancies that might pose a risk in case of litigation. - Thank you!