As a first note, employers have always been required to provide a safe and healthy workplace.
The Occupational Safety and Health Administration (OSHA) is the federal agency responsible for overseeing and handling complaints regarding workplace safety matters. OSHA has provided guidelines during the pandemic. Employers are required to abide by the guidance from OSHA, the Center for Disease Control and Prevention and state agencies to avoid liability. Employers should implement a pandemic response plan that documents what steps it is taking to comply with the various state and federal requirements, e.g., social distancing, cleaning of work areas, the wearing of masks, and so forth. Employees need to be trained on these requirements.
If an employee believes that an employer is not complying with guidelines, the employee should notify the employer. The employee may also notify OSHA, and state and federal whistleblower laws should protect the employee from retaliation.