Vouching of Advances to suppliers can be done as follows:
Obtain schedule of debit balances in trade payables‟ account and pay particular attention to the age of the balances. Also scrutinize the bought ledger.
Enquiry should be made for long unadjusted outstanding and check as to whether any of them would require provisioning.
Examine that the advances have not been shown as deposits in balance sheet.
Confirmation of balances should be obtained and reconciliation be done in case of any discrepancies.