Any HDD made since the mid-late 90s has an onboard temp sensor. You just need a program that can read the SMART data to read it.
http://www.seagate.com/support/disc/.../en/smart.html is an online version, you can find various tools about the net to read it without using a net connection. Note, most don't work for drives on a raid controller, however the better one's do work on SCSI disks.