The content validity of survey instruments, like psychological tests, is assessed by overview of the items by trained individuals and/or by the individuals from the target population. The individuals make their judgments about the relevance of the items and about the unambiguity of their formulation.
The major distinction from the face validity is that the individuals using for evaluation are either experts in the field or belong to the target population - i.e. not chosen casually.
The content validity, like the face validity, is a qualitative measure of validity; it is normally not quantified with statistical methods.