I usually search through google, you may also want to try consumer reports as well! You can also use ask searches, such as yahoo ask community where other people answer questions for you
I go to a reputable retailer (Best Buy, QVC, Etc.) and look at the consumer feedback on the said item. Typically, this is good to see what possible problems the unit in question might have. You can also try doing what Shinra Soldier said. =p
I usually just look at Customer Reviews on Amazon since they used the products and there are a lot of reviews on products there. I tend to avoid buying products with very low scores by the customers since the products must have problems if 70-90% of the people who bought them gave them a low rating.