There are many resources available that help HR stay up to date with federal payroll laws, including:
The Wage and Hour Division (https://www.dol.gov/agencies/whd) of the Department of Labor provides information about labor and payroll laws and changes.
Posters. These are federal labor law notices(https://www.dol.gov/general/topics/posters) that the employer has to post in the area frequented by the employee. They are free and available on the government website forms.
Employment law attorneys. Every company should have access to an employment law attorney who helps in resolving conflicts and provides information about changes in labor laws.
Professional associations like the Society of Human Resource Management (SHRM-http://www.shrm.com/) provide information related to current labor laws for each state. It's wise to become a member for their many benefits and subscribe to email lists on topics like payroll laws.
Privately-owned websites like HR.com(https://www.hr.com/) contain a lot of features like discussion forums and news updates for members.