You would know if you were paying PPI, there is always a line on your statement showing PPI being added if it applies to you (although some companies call it something else). On my statements, it's the line above the last one where they add the interest and/or any fees.
We are using a company because it's convenient for us - not allowed to advertise here though I believe - they take a percentage of the final recovered amount but so far they are very efficient.
What you need to do is to find an old statement from each of your cards, showing that PPI is being charged - whoever claims it back (you or an intermediary) you will need to send in a copy statement showing that in the past, it was charged.
Believe me, if you are recovering £1000 (say) it's worth spending a tenner to get a copy statement, especially if you can show one with a sizeable balance and a correspondingly large amount of PPI being charged.