A Project Manager is expected to know HR Law. The problem is that most PMs rely on word of mouth HR training. HR law changes almost monthly, because of this a PM should always seek out the most recent HR laws applicable to the situation at hand. A companies HR department may be a good starting point, but it is not a guaranteed solution. HR departments can often be overworked and understaffed. Best course of action is to always verify what the HR department tells you with a quick review of some key sites.
http://www.dol.gov/index.htm – This is a sight I believe PMs should review monthly. It contains the basis for federal US law. It is always a good idea to start a review of a HR question with federal regulations. States may have stricter laws but they all must comply at least with the basics of federal law. It has happened a few times where state law was not as strict as federal law, but a majority of those cases end up with the courts imposing the federal law as the standard.
http://hr.blr.com/ – This is a great site that will give you a breakdown by state and a summary of the federal law. I recommend going to the state government HR site only if the issue at this point is not clearly resolved. State HR sites generally host the actual law with no interpretation. This means you will need a lawyer often with HR experience to really know what to do.
Having run many international projects there are two pieces of guidance I give when dealing with other countries. First US federal law is the safest guideline to use as a basis when dealing with HR issues if you are working for a US company. Always error on the side of caution. HR issues that are global are difficult and there is no simple place to go for answers. Personally I use Universal Declaration of Human Rights as a guideline. If you find that someone is in violation of the Universal Declaration of Human Rights you should take action as a Project Manager to get the situation resolved.