How do I prove to the IRS that I'm entitled to receive information about my parents' taxes?
You should be able to complete and file IRS Form 56, “Notice Concerning Fiduciary Relationship” and indicate that there is a valid trust. You can download the form directly from the IRS website.
Part II will ask you to indicate your authority -- and you should check "Valid trust instrument and amendments."
Form 56 comes with a set of instructions and warnings about how to complete it, but if you have questions, your CPA should be able to answer them based on his knowledge of your situation.