In the Release Notes for the SPFx (https://github.com/SharePoint/sp-dev-docs/wiki/Release-Notes-GA) we can read "... We will start with the same first-release tenants (not users, tenants) that have SPFX today. Over the course of a couple of weeks, it should roll to all tenants. You will know if it has reached a given tenant because uploading the package into the app catalog will say it is valid. ...".
As far as I know this is the way to check... @Dan Holme Do you know a different and better way?