For PPI on loans or mortgages, you will probably be able to find signs of PPI payments on your original agreement. For PPI on credit cards, it should show up on your statements. Generally it would have been charged monthly.
Remember to check for the different names PPI was sometimes called on your documents, such as: ASU insurance (Accident, Sickness and Unemployment), Life & ASU insurance (Life & Accident, Sickness and Unemployment cover), PLP (Personal Loan Protection), CCRP (Credit Card Repayment Protection) and MPPI (Mortgage Payment Protection Insurance).