How to monitor space usage on ASM diskgroups

Just use asmcmd. For example:

[oracle@oel61 ~]$ . oraenv
ORACLE_SID = [+ASM] ? +ASM
The Oracle base remains unchanged with value /u01/app/oracle
[oracle@oel61 ~]$ asmcmd lsdg
State    Type    Rebal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Voting_files  Name
MOUNTED  EXTERN  N         512   4096  1048576     30708    28479                0           28479              0             N  DATA/
[oracle@oel61 ~]$ 

Or get the info directly from the ASM Oracle instance:

[oracle@oel61 ~]$ . oraenv
ORACLE_SID = [+ASM] ? +ASM
The Oracle base remains unchanged with value /u01/app/oracle
[oracle@oel61 ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.2.0 Production on Thu Mar 7 10:44:44 2013

Copyright (c) 1982, 2010, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Automatic Storage Management option

SQL> SELECT name, free_mb, total_mb, free_mb/total_mb*100 as percentage 
     FROM v$asm_diskgroup;

NAME                  FREE_MB   TOTAL_MB PERCENTAGE
------------------------------ ---------- ---------- ----------
DATA                    28479      30708 92.7413052

SQL> 

Try this...

col gname form a10
col dbname form a10
col file_type form a14

SELECT
    gname,
    dbname,
    file_type,
    round(SUM(space)/1024/1024) mb,
    round(SUM(space)/1024/1024/1024) gb,
    COUNT(*) "#FILES"
FROM
    (
        SELECT
            gname,
            regexp_substr(full_alias_path, '[[:alnum:]_]*',1,4) dbname,
            file_type,
            space,
            aname,
            system_created,
            alias_directory
        FROM
            (
                SELECT
                    concat('+'||gname, sys_connect_by_path(aname, '/')) full_alias_path,
                    system_created,
                    alias_directory,
                    file_type,
                    space,
                    level,
                    gname,
                    aname
                FROM
                    (
                        SELECT
                            b.name            gname,
                            a.parent_index    pindex,
                            a.name            aname,
                            a.reference_index rindex ,
                            a.system_created,
                            a.alias_directory,
                            c.type file_type,
                            c.space
                        FROM
                            v$asm_alias a,
                            v$asm_diskgroup b,
                            v$asm_file c
                        WHERE
                            a.group_number = b.group_number
                        AND a.group_number = c.group_number(+)
                        AND a.file_number = c.file_number(+)
                        AND a.file_incarnation = c.incarnation(+) ) START WITH (mod(pindex, power(2, 24))) = 0
                AND rindex IN
                    (
                        SELECT
                            a.reference_index
                        FROM
                            v$asm_alias a,
                            v$asm_diskgroup b
                        WHERE
                            a.group_number = b.group_number
                        AND (
                                mod(a.parent_index, power(2, 24))) = 0
                            and a.name like '&&db_name'
                    ) CONNECT BY prior rindex = pindex )
        WHERE
            NOT file_type IS NULL
            and system_created = 'Y' )
WHERE
    dbname like '&db_name'
GROUP BY
    gname,
    dbname,
    file_type
ORDER BY
    gname,
    dbname,
    file_type
/