If you're talking about within the disks utilized by ASM, it's most likely that you don't have any unallocated space. You can check with cellcli on one of the storage servers to see if any of the celldisks have any free space:
list celldisk attributes name,freeSpace,size where diskType = harddisk detail
It will also tell you if you have high performance or high capacity disks. To see whether you have free space within your ASM diskgroups, run the "lsdg" command from the asmcmd utility on one of the database servers. Look at the Usable_file_MB column for each diskgroup.