Non-functional requirements are notoriously non-testable! By this, we mean it is more typical that non-functional requirements are fuzzy/less-clear. In a simplistic form “The system should be robust” is non-testable i.e. It is definitely not clear as how to validate this!
Rather than identifying non-functional requirements and describing them, it is suggested that we look at each requirement and partition these into functional and non-functional aspects and probe into the key attributes to be satisfied for the requirement. For attribute, GQM (Goal-Question-Metric) of core concept of STEM enables deriving metric(s) to ensure that each attribute is indeed testable. Later the various similar attributes across all the requirements can be aggregated to create the system-wide non-functional requirement.
In this manner non-functional requirements are clearer and testable.