How to check if your insurance agent mis-sold a policy

Insurance mis-selling often comes to light only after a few years. Someone reviews their finances, tries to stop paying premiums, or checks the expected returns and suddenly realises the policy works very differently from what the agent had explained.
Sometimes a policy that depends on market performance is described as a guaranteed investment. In other cases, the buyer is told they only need to pay premiums for a few years, when in reality the commitment is much longer.