You can use a number of programs, like the Asus 'Probe'. Motherboard Monitor 5.0, Sisoft Sandra, CpuIdle, Rain. There's more, but those are the only ones I know of.
Just be aware that some programs (Probe) will not give you a 'true' reading, but feed the temperature into an algorithm, and report back a 'core' temperature. I perfer the raw data myself, not data that's already been interpreted....