Is there a way to check that the opt-out policy has been applied ?

Hello.

You can check the applied opt-out policy by using the "DescribeEffectivePolicy" API as described in the document below.
How about running this periodically and being notified if there is no policy?
https://docs.aws.amazon.com/organizations/latest/userguide/orgs_manage_policies_ai-opt-out_effective.html#how-to-view-effective-tag-policy