How would you determine any of your staff members performance at work whether you are there or not there in the department?

There can be many staff checks which can be devised as per the circumstances to check the performance. He can be told to verify the guests and record his observations and actions for each.

You need to adopt a "management by objective" methodology where employees are fully aware of what is exepected from them in terms of performance and results.  It would even be better to get the empployee concensus (agreement) to these objectives when you set them with the employees

Secondly, you need to adopt an effective monitoring, reporting and measurement system to see how the employees are actually performing versus the objectives set for them.Last but not least, you need to adopt a "multu-path" feedback reporting system from the employees themselves, from their colleagues/peers, from their clients/customers, and from other management ranks within the organization.  This could give you a360 degree view of the employee performance challenge !