If you log into cloud.oracle.com and go to the ADB cloud console page, then click on any of your Autonomous Database instances. This will take you to a page showing all the information about that specific instance. At the top of the page there is an information box which will tell you the database version.
You can also just log into you cloud database and run a select on v$version view.
...and as of today (Oct 31) you have an additional option when creating an Autonomous Database:
there is more information here: https://blogs.oracle.com/datawarehousing/getting-a-sneak-preview-into-the-autonomous-world-of-19c
If you want more information about preview versions for Autonomous Database then checkout the overview page in the documentation which is here: https://docs.oracle.com/en/cloud/paas/autonomous-data-warehouse-cloud/user/autonomous-preview.html