Custom series that have been added should be done with client Defined Prefix, ex. ACME Final Forecast, so they are can be identified as non-standard series. With that said, it is not always done, so beginning in v7.3.0 there is a new column called Application_Id in the Computed_Fields table. This is the table that stores the Series information.
Standard Series will have have a prefix similar to COMPUTED_FIELD, while a custom series will have an Application_Id value similar to 9041128B99FC454DB8E8A289E5E8F0C5.
So a SQL that will return the list of custom series in your database might look something like this:
select computed_title Series_Name, application_id
from computed_fields where application_id not like '%COMPUTED_FIELD%' order by 1;