The best way to make managers aware of these needs is through training. The training must be provided and it must be reinforced on a regular basis. Of course, this is not guaranteed to work. It is possible for managers to receive regular training and still make mistakes. However, there is no way to prevent all mistakes from happening.
Training would need to have at least two parts. One part of the training would need to focus on factual matters. In other words, this part of the training would focus on imparting factual information about the rules and laws that have to be followed in the staffing process. This information would need to include information about the sanctions that can be meted out on a company if it does not follow the legal requirements. A second part of the training would be more applied. In that part of the training, managers would do things like case studies and role playing. The point of these exercises would be to give them the chance to apply the factual information they have been given to scenarios that might occur in real life.
This sort of training is the best way to make managers aware of the legal requirements that go along with staffing and with the consequences of making mistakes.