SQL Statement

select tablespace_name, (2*max_extent) - max_free from
   (select s.tablespace_name, s.max_extent, free_space.max_free
   from (
      select tablespace_name, nvl(max(next_extent),0) max_extent
      from dba_segments
      group by tablespace_name) s,
      (
      select   t.tablespace_name, nvl(max(f.bytes),0) max_free
      from     dba_tablespaces t, dba_free_space f
      where    t.tablespace_name = f.tablespace_name
      group by t.tablespace_name) free_space
   where s.tablespace_name = free_space.tablespace_name
   order by tablespace_name)
order by tablespace_name

Description

The alert will list tablespaces that contain segments with a next exent size * 2 that is larger than the largest chunk of free space available in the tablespace.  In other words, the segment will not be able to extend in the near future if it is growing.