You usually think of validation (test) activities as occurring near the end of a development process, and indeed that is usually the case. It's hard to validate something that hasn't yet been built.

Some of these activities are done by software developers, including functional verification tests (FVT), system verification tests (SVT), stress (load) tests, and long-run tests.

The validation activities that typically include participation by VUI designers are SVT, customer acceptance tests (CAT), grammar tests, usability tests, and A/B tests.

System Verification and Customer Acceptance Tests
The purpose of both SVT and CAT is to make sure the system is working as designed. VUI designers can play a role in recommending test cases and in participating as testers. Due to their deep knowledge of the intended design, they can detect problems that might otherwise slip through the cracks, such as problems in the connection points of audio segments, incorrect prosody in audio segments, and timing issues.

Although CAT should be driven by clients, in practice they may rely on the design team to help them design and execute the test. SVT typically precedes CAT, so problems discovered during SVT should already have been corrected, providing another chance for VUI designers to ensure the system is working as designed.

Grammar Testing
See How to Pick Synonyms.

Usability Testing
See Usability Testing.

A-B Testing
See A-B testing.