bank audit

Check bank statement which includes entries for cash deposits.

If you are asking for a random transaction, check the deposit slip attached in the vouchers, which will be preserved by the bank. Cross verify the same in the CBS (accounting software of the bank)
(other part that you can verify alongwith this is of levy of cash handling charges, if any)
And if your question is to verify the balance of cash held by the bank (branch)
so a surprise verification on a day is a good way to proceed
Take the count of physical cash and tally it with system..
(Alongwith this process you can concurrently verify the compliance on other areas such as clean note policy, rentention limit, insurance for cash, dual lock system etc.)