If you need to test if a datafile is empty you usually need to query DBA_EXTENTS view to check that the related datafile has no extents. This works well but can be slow for several reasons; see My Oracle Support for Oracle Database release 11.2.0.3 Query against DBA_EXTENTS slow after upgrade to 11.2.0.3 (Doc ID 1453425.1).

On an empty database (i.e. a database just created with DBCA with no application schemas) a simple query using DBA_EXTENTS takes about 4 seconds:

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE    11.2.0.3.0      Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production

SQL> --
SQL> set timing on
SQL> --
SQL> create tablespace test;

Tablespace created.

Elapsed: 00:00:05.00
SQL> --
SQL> column empty format a5
SQL> select file_id,
  2    'yes' as empty
  3  from dba_data_files df
  4  where not exists
  5   (select null
  6    from dba_extents e
  7    where e.tablespace_name = df.tablespace_name
  8      and e.file_id = df.file_id);

   FILE_ID EMPTY
---------- -----
         5 yes

Elapsed: 00:00:03.96
SQL>

On a non-empty database this query may takes a couple of minutes depending on the number of extents among other factors.

Another way to test if a datafile empty is to check if the datafile free space size is the same as DBA_DATA_FILES.USER_BYTES which is the size of the file available for user data:

SQL> --
SQL> select file_id, bytes, user_bytes
  2  from dba_data_files
  3  where tablespace_name='TEST';

   FILE_ID      BYTES USER_BYTES
---------- ---------- ----------
         5  104857600  103809024

Elapsed: 00:00:00.00
SQL> --
SQL> select sum(bytes)
  2  from dba_free_space
  3  where tablespace_name='TEST';

SUM(BYTES)
----------
 103809024

Elapsed: 00:00:00.00
SQL> --
SQL> column empty format a5
SQL> select file_id,
  2    'yes' as empty
  3  from dba_data_files df
  4  where user_bytes =
  5   (select sum(bytes)
  6    from dba_free_space fs
  7    where fs.tablespace_name = df.tablespace_name
  8    and      fs.file_id = df.file_id);

   FILE_ID EMPTY
---------- -----
         5 yes

Elapsed: 00:00:00.05

Compared to DBA_EXTENTS, DBA_FREE_SPACE is likely faster especially for an empty datafile because the number of free extents should be small.