A CPU can run fine from anywhere up to around 55 C. After that, you should probably be careful. I know that they can handle up to around 90 before they actually get destroyed, but for optimal performace, stay below 55. I have a 2500+ Barton, and that runs pretty warm, although it never goes above 50. I need a better cooler on it.
There are 3 different ways to monitor temps. One is by the built in sensor on the processor, that the motherboard reads and it shows it in the BIOS. Programs like Motherboard Monitor, and PCAlert will show you the temps from the BIOS. Another way is to mount a thermal sensor under your processor and mount it to a digial gauge. Or you can take that sensor, and tape it right next to the processor core. I put mine next to the core, and I do get a different reading than the onboard sensor. You have to be careful though when using the sensor next to the core, because you want to make sure it is not interfering with the heatsink.
You can pick up these sensors at www.directron.com
or maybe at www.newegg.com