How to check customers who have opted out to email subscription

[tyamaguchi]

Is there an easy way to see who among the external customers have opted out form ticket email subscriptions?

My team has cases where customers have clicked "turn off email notifications" whether intentionally or accidentally for a given ticket. Just as part of support project administration, I'd like to see how we can check (run a report) who opted out to any of our tickets, while there are many tickets to consider.

Thanks!