To verify RAID is working:
cat /proc/mdstat
You should see each drive assigned to an array and the drive/partition number. If you have two drives you should see [UU] meaning both drives are uniform. If you see [U_] or [_U] it means that one of the drives has failed (respectively).
To check for free space:
df -h
This will display the array/drive and it's used, free, and % capacity in human readable numbers (-h).
If you have any other questions let me know,
Nathan