RAID1 just does mirroring. That is, you have two hard drives, every piece of data that you send to or that gets written to the server goes to both hard drives. This way, in the event that one hard drive fails, the other hard drive is still there with all of your data.
As far as the server is concerned, it will only "see" one hard drive because there is no way to write data to one hard drive and not the other.
You will need to install RAID monitoring for whatever RAID controller you are using to monitor the health of both drives. Contact your datacenter for more information about this.
(This is assuming hardware RAID is being used)