Believe me, you would know if you had a limited stay. You would have received a visitor record as stated by other members and a blue stamp with date stamped in it when you arrived and underneath the stamp would have had the date you needed to leave. Mine also had the file number of the visitor record that was given to me. I was given 4 1/2 months to stay. This was my first ever time and only time coming to Canada.
When you were told by the Border Services, CIC and us on the forum that a black stamp with no date means 6 months, then that is what it means. The officer did something wrong and you got lucky. Embrace the extra 3 months you got. If you are still worried either go back to the airport to ask someone, call CBSA and CIC again, contact a lawyer/consultant, or order FOSS notes. FOSS notes if faxed in can take 2 weeks to get or a month if mailed in. The FOSS notes are the comments/notes the officers put into the computer based on previous visas/visitor records, permits, border crossings, extensions, and processing of PR app in Canada. This may be valuable to truly finding out what the officer put into the database. I still think you should rest assured that you have 6 months to stay. If he wanted to give you only three months than he should have given you a blue stamp and visitor record but since he didn't then his mouth didn't match his actions.