Leave it on for a while to give the disks a chance to heat up, then reboot the system, disable RAID in the BIOS, and boot the system off of a Linux live CD. You should be able to use the SMART tools available in Linux to read the temp sensors even if the system is not bootable from the hard drives.
Alternatively, just open the case after the system has been on for a while and feel the drives. If they're anything more than just warm to the touch, you need to beef up the ventilation.
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