Whilst the new rules suggest the banks and lenders will have to contact all past PPI policy customers there is no guarantee they will do so. Therefore the best thing to do if you've had a loan or a credit card in the last six years is to check your agreement to see if there is a PPI policy attached to it. Don't just rely on looking at your statement either as often the PPI payments are front-loaded at the start of the repayments.
If you can't find your original loan or credit card agreement then you can contact your bank or lender and they are legally obliged to send you copies of your agreement, although some may charge you a small admin fee (no more than £10 maximum). Here is a list of banks and lenders contact details for PPI's.