S.W.
I have worked as an HR Manager. HR does not exist for your confidentiality. They are required by law to reveal much of the communication that comes their way. Employees should always ask first if their question or request subject can be kept confidential before making a request. The HR person can then tell them if they can keep it confidential or not.
First I would research all company policy documents. Then, if I still couldn't find the info, I would make a phone call with the question "What is the company policy about number of work hours per week to receive full benefits?" That should be a simple enough question to be answered over the phone and should not trigger a communication to your supervisor.