Requirements validation is necessary to make sure the Requirements are of a high standard, suitably define the Customer's problem (or opportunity) and are sufficient for the implementation teams to design and implement the product. It is imperative that the requirements have the desired level of quality and are complete and necessary. There are a number of ways that Requirements can be validated, but probably the two most common ways are to perform team reviews and to assign test cases to the requirements.
The team reviews are typically conducted by team members or other analysts who have some familiarity with the domain, but were not themselves responsible for the requirements development or management. Enterprise Architect has a handy tool to assist with this process, called the Formal Review, which works across the entire model and allows reviewers to record their findings in discussion documents and to reference model elements. There is also a Requirements Checklist element available from the 'Extended Requirements' page of the Requirements Toolbox, which provides a useful mechanism for checking the quality of Requirements.
Test Cases can be defined at a number of levels from User Acceptance tests down to Unit tests. Defining the test cases early in the requirements development process creates a double check on the Requirements, because when test cases are defined issues with the Requirements are often uncovered. Enterprise Architect has a number of facilities to define test cases and a modeler can select whichever is the most appropriate for the endeavor.