Separate the responsibilities of your employees to reduce theft. Some examples include:
One person handles cash at the business, while another records transactions in accounting records
Reconcile the cash every month. Compare the recorded transactions to the amount of cash in the store. This is done by someone who did not record the transactions.
Keep cash locked at all times.
Reference: Rogers, JC. Internal controls are the key to preventing theft in small businesses, https://www.jcrogerscpa.com/Internal%20Controls.html