Basically, you need to go and look at the science. Go and read the IPCC reports (Intergovernmental Panel on Climate Change), see what the National Academy of Sciences, the American Meteorological Society, etc have to say.
If you listen to the pseudo-scientific groups (be they sponsored by the right or left) you're not going to get good science. Avoid Michael Chricton like the plague. Don't read things sponsored by the American Enterprise Institute or anything by any other "think tank"
Also, there are many other things that make the IPCC not exactly a source that is 100% trustworthy.