A good accountant should be able to show you ways to catch a thief. This process looks for things like sequentially numbered invoices or verifying revenues to bank deposits. Do Not Assume any employee or partner is beyond theft. Personal circumstances change and so does motivation to steal. Look for Spikes in Expenses that appear to be excessive.
Accountants are good here.
Theft Loss Prevention Forms are very helpful. I like the form used in the software Employee Manual Maker. This form is to be signed by each employee. They get one copy and the other copy goes in the employee's personnel file. The forms states clearly the causes for dismissal.