Have you read Erin's blog article explaining the difference between Angels and Spirit Guides? It's good stuff. Spirit Guides, Guardian Angels, and Archangels Defined
How do you know you're communicating with Spirit Guides and not some other negative entity? I've seen this answered on many blogs, and basically you'll know from how the info received feels to you. Both Spirit Guides and Angels have our best interests at heart and give info from a non-judgemental place of love. They want to help us and want us to live fuller and happier lives.
If you raise your vibration before attempting communication, you'll be protected from the negative entities anyway. As a Catholic (and I know where you're coming from), you're more comfortable with the concept of Angels. So ask your Angels to send you signs/synchronicities about whether your Guides are real or not. Ask them to help you discover the best way to communicate with them. Ask your Angels to protect you from any and all negative associations during Spirit Guide communications and they will certainly help you.
I think people who are gifted with being able to communicate very easily with Spirit Guides, Angels, and other beings have all gone through a period in their intuitive/spiritual development where they have encountered negative entities. They definitely know the difference and can tell us less naturally gifted people how to avoid them. That's why I take the advice of people like Erin so seriously when they say it's important to raise your vibration before communicating with Guides.