In Rules of District Court Order 50(1)(3)(iv), how to verify that the interest to be charged is owned beneficially by the judgment debtor?

In Rules of District Court Order 50(1)(3)(iv),  how to verify that the interest to be charged is owned beneficially by the judgment debtor?  How to prove the debtor has contributed whole or part of the purchase price of the flat?  If the mortgage had paid off long time ago