Is there a way to find if tablespace is currently being used?
The tablespace will be in use when the database is up and all the datafiles will be stamped during a checkpoint.
What is your question? Do you want to find out if any users are using it by creating data in some of the tables which are in that tablespace? Then you can analyze the tables in that tablespace and check the avg space used. If that is changing or the table size is changing then the tablespace is in use.
As Baliga told, all the tablespaces which are online are in use.
If you are wanting to see the status (online or offline) of the tablespace, use the following query:
select tablespace_name, status from dba_tablespaces;
If the tablespace is offline is not available for data access.
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