How do I make sure that all petty cash transactions get recorded?

It is your responsibility as a business owner to set up policies for use of petty cash, to monitor petty cash transactions, and to make sure that employees are using petty cash responsibly. Some possibilities:

Give trusted employees a business debit card and require them to turn in details on business purpose for all their debit purchases.

Require employees who use their own cars for business to have a tolling tracker. You can provide it and get the details. You will need the employee to verify business purpose.

If employees have smartphones for business use, require them to include metaCASH to capture receipts by photo or to send them to your bookkeeping department.

Best Option is Subscribe to MetaCASH for Easier Automation