How to check how much disk space is allocated/ in-use by an ASM instance?

Login to your +ASM instance (SYS AS SYSDBA) and execute the following query:

SQL> COL % FORMAT 99.0
SQL> SELECT name, free_mb, total_mb, free_mb/total_mb*100 "%" FROM v$asm_diskgroup;
NAME                              FREE_MB   TOTAL_MB     %
------------------------------ ---------- ---------- -----
DATA                               917104    1482145  61.9
RECOVER                             17387      17437  99.7

From Oracle 10g Release 2, one can also use the asmcmd command line utility:

ASMCMD> du
Used_MB      Mirror_used_MB
   1523                1523

ASMCMD> lsdg
State    Type    Rebal  Unbal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Name
MOUNTED  EXTERN  N      N         512   4096  1048576     11264     9885                0            9885              0  DISKGROUP1/
MOUNTED  EXTERN  N      N         512   4096  1048576     10240     9906                0            9906              0  FLASH/