I think, depending on what graphics card you have, there's software that can monitor the temperature of it, and I think there's programs to monitor other components (is it CPU-z or something?). Someone else will have to answer that.

In order to check what wattage your PSU is, you'd have to open up your case and have a look at it, it should say. I'm not really sure how you'd check the voltage otherwise. There might even be software that does it.