If the part-time worker seeks coverage for medical insurance from the Marketplace when the employer should have offered insurance coverage, the employer is then at risk for penalties for failing to comply with the ACA.  The only way to document or prove the employee worked LESS than 30 hours average per week is to track and document the employee’s actual hours worked.  If there is no documentation of hours worked, then the employee has the advantage, since under ACA and employment laws, it is the employer’s responsibility to monitor all employees’ hours worked.