I check space every week, and had noticed that one of the disk groups was growing much more than the others.
So I logged into OEM, navigated to the ASM instance (use the search field), and selected 'Automatic Storage Management / Disk Groups'.
This showed me a bar chart of the disk space usage
Click on the name that has the most used, in this case RECOC1.
There is a chart at the right of the page that shows the database that is using it.
In this case the DEVDG database.
To see the files that are responsible, click on the 'files' tab
You can then expand the directories and see where the files are
In this case, ARCHIVELOG files going back a few months
The archived logs are not being deleted. Further investigation showed that this was a data guard database and had not been backed up, and the Primary was not set to delete the logs after they had been backed up and applied.
This took me about 10 minutes in OEM.