Flag user across multiple domains

Hi guys,

We went for multi-domain setup and have a problem.

All we want to do is have native subscription dialogs on multiple domains and flag users that are subscribed on domain a.com and have done action but are not subscribed on domain b.com

the flow user would take:

  1. visits domain a.com and subscribes to domain a.com with native dialog
  2. visits domain b.com does an action
  3. We want to check if he has domain a.com subscription and flag him during visit on domain b.com
  4. We might want to also show him native subscription dialog on domain b.com

What is the best approach to tackle that problem?