I don't know that there are any staple beliefs of the Presbertyrians. I do know that they meet to discuss doctrine periodically, and change it. As a result, many Presbertyrian churches teach unbiblical doctrines or fail to teach Biblical ones. Sometimes they may lend support to a wierd political faction in some banana republic. For these reasons, my grandparents left the denomination. Granted their particular church still taught the Bible, but Grandpa had a prison ministry, and when inmates asked him what kind of church he went to, so they could find a similar one, he'd cringe.
So, as with all churches, you need to really watch what they teach, make sure it's Biblical. Take your Bible, follow along, pray about the lessons. Think about them and see if there are any discrepancies between Scripture and the teaching. For example, I hear that many Presbertyrian churches are embracing gay marraige. That's not Biblically right at all, so if your church teaches that, it's time to find another one.