Manoj is spot-on about sending the email to test the preference center. One thing to note is that the preference center link will only work when launching a campaign or Canvas. Sending a test email directly from the Braze dashboard will not generate a valid link, so you'll need to send the actual email to yourself.
After sending the test email, click on the preference center link to verify that the page displays correctly. Check that all subscription groups are listed and that the styling matches your brand guidelines.
If you want to test subscription changes, make changes to the subscription preferences and save them. Verify that the changes are reflected in the user's subscription status in the Braze dashboard.
And then to ensure that the preference center is filtering emails correctly, you can create a test campaign targeting the subscription groups. Send the campaign and verify that only users who are subscribed to the relevant groups receive the email.