Here's a way to check it
[root@alpha /root]# cat /etc/raidtab
raiddev /dev/md0
raid-level 1
nr-raid-disks 2
chunk-size 64k
persistent-superblock 1
#nr-spare-disks 0
device /dev/hda1
raid-disk 0
device /dev/hdb1
raid-disk 1
raiddev /dev/md1
raid-level 1
nr-raid-disks 2
chunk-size 64k
persistent-superblock 1
#nr-spare-disks 0
device /dev/hda5
raid-disk 0
device /dev/hdb5
raid-disk 1
Also you should see something about the partition being a mirror in dmesg after boot.
I would put mine here but I have not rebooted for quite a while.
