You can quickly check the status of your application to join a Community by going to the Find a Community page within your Communities dashboard. Once there, click Open Communities Requests.
This list shows you the requests that have not yet been approved or rejected by the Community admins.
If you requested to join a Community, but it's not on this list, it means your application to join was either approved or rejected. If you were approved, then you should have access to it and see it on the Your Communities page.
If you don't have access to it and it's no longer in your Open Communities Requests tab, then unfortunately you've been rejected for that Community. But don't give up!