Meet and greet. When you suspect someone, make eye contact. If you feel comfortable, touch people briefly and lightly on the arm as you speak to them. Untidy merchandise encourages theft. You are most vulnerable during staff breaks, high traffic periods, and opening and closing.
Body language is a good indicator.
Short training sessions from a professional can give management and staff the skills to readily identify shoplifters. The goods may be put back. Only apprehend outside the store. All the above advice is dependent on human factors, therefore prone to weaknesses. Deterrent systems range from ineffective to excellent.