Refer to Appendix B of the Specification Handbook where you will find a section on Assessment Guidance. Candidates can demonstrate programming skills, and ability to interpret the code for the CNC machine through on-screen simulation. Where code is produced they can also show this by directly interpreting the code (e.g. G code). A portfolio showing step by step annotated screen shots is a good way to demonstrate this.
It is recognised that with some types of CNC machine (e.g. rapid prototyping and laser cutters) that no visible code is produced, so the use of on-screen programming and simulation can be used.