After a boot disk crash, I wanted to make sure existing data disks were unaffected. We use Oracle ASM for all our bare metal database servers and in this case there wasn't a proper backup of the (development) database. ASM marks its disks as being part of an ASM disk group and checking the first block of a partition. This lets you verify that the data still exists.
Upon reinstallation of the OS and ASM software, you can then reuse existing ASM disks and groups and restore the data.
To verify the disks are still marked as ASM disk groups, check the first block of the partition:
dd count=1 if=/dev/sdc1 | od -c
sdc being our first data disk after local disks (sda) and the boot LUN (sdb). Note the text string on the third line in capital letters and the string " D G _ D A T A 1", naming the disk group. You can check the list of available partitions using 'cat /proc/parititons|less'.