Without more details we will have to assume that you are using Hardware Raid. When you enable Hardware Raid the controller abstracts the device and presents a virtual device to the OS.
Dell Server, two disks without Hardware Raid enabled Linux would see them as /dev/sda and /dev/sdb. You could then use smartmontools to query /dev/sda or /dev/sdb. If you build a Software Raid device in Linux (/dev/md0) you will still need to query the devices directly (sda, sdb) rather than the Raid device (md0).
If you do use the Hardware Raid functionality, the controller will present a device (sda) which is similar to the Software Raid device and you will not have the ability to query the SMART functions of each disk directly as each disk is hidden from the OS.