How do you validate messaging after the product has been launched?

Talk to your sales and CS people. The folks in your organization who are repeating the messaging to prospects/customers will have a lot of feedback right away. You can also use tools like Gong for this. We are setting up a Revenue Advisory Board who will help give feedback while we are developing the messaging then give feedback after using it in the wild

Quantitive metrics (see above answered questions) such as A/B testing, Website data, etc.

Combined with Qualitative feedback:

• Sales feedback

• Customer feedback

• Analysts feedback

• Social channels