Any document evidencing the grant of money from you to any other person is enough to prove the debt. Any document, contract, message, telephonic conversation, mail or post can serve as a proper evidence in Court of law to prove your debt. Any witness or any other circumstance arising from the consequence of such debt can also help you prove the debt. Bank account transactions, promissory notes, a contract or other payments so made serve as legal evidences of the lending of such money.
A good debt recovery advocate in India can help you better in establishing your case in a Court of law.