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&gt; 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&gt;