How do you approach a employee who is stealing?

The answer depends on a couple of things. First, what are your objectives? Do you want to counsel, fire, or criminally prosecute? And second, do you already have sufficient evidence to take action, or is your investigation not complete?

Once you define your objectives, are you talking to the employee to gather information, or simply to inform them as to your decision regarding disciplinary action?

If you have sufficient information to counsel or fire, I suggest you handle it no differently than you would any other counseling session or termination of employment. State the problem and corrective action, or state the problem and notice of termination. Your employee will attempt to rationalize - don't indulge too much of that.

If you want to prosecute, do not approach them at all. Contact your local police and show them that you have all the information needed: an identified suspect, and evidence conclusively linking them to the theft. They will take it from there, if you do in fact have a completed investigation. If not, they may or may not actively work your case based on its priority and available resources.

If you do not have sufficient information for termination or prosecution, you will need to develop it yourself first, before you approach anyone. Also, confrontation of an employee by a person who is not trained and experienced in interview will only elicit a denial - build your case first. Working with a professional investigator who is trained and experienced in this specific type of case may be an option to ensure you build a sound case without incurring civil liability.