How can I ensure that a platform will have the required performance with multicore interference taken into account before verification?

You cannot know the performance of multicore software considering interference without determining and testing the interference channels. However, it is very important to de-risk the performance not being sufficient when selecting a platform and software stack.

These considerations are explained in detail in the MACH¹⁷⁸ Foundations white papers Processor Selection Considerations, RTOS Selection Considerations, and Software Architecture Considerations.