There is no software utility you can use because there is no hardware to measure the temperature. So you need to get the hardware. If your motherboard has pins for it you can get a thermal diode and plug it into your motherboard and use the motherboard software to monitor it. Otherwise you can get front panel displays connected to a thermal diode which will monitor the temperature for you.
I don't think you have to worry too much about seagate SATA drives heating up. Unless there is no fan in the case and they are right up to one another. RAID or no RAID the tempetures will be about the same. Unless you have 10,000 or 15,000 rpm hard drives you don't really have to worry much.