Search Google using the main keywords and brand names that you intend on advertising. Are there any reviews? What about scam reports and complaints?
Search for the product on Amazon. Read what the reviews have to say. What are the main questions about the product? These can become ideas for your ads and landers.
Look for competing products. Is the offer best in class? If it's not, don't bother to run it. A perfect example was this "military grade super bright flashlight". It's a piece of crap. A couple of quick searches revealed that most flashlights in that price range are brighter and more durable.
Also quickly check their BBB (Better Business Bureau) status. Do not do business with a company that's poorly rated or has a lot of complaints. If you do, most of your commissions will be reversed due to refunds, defective products and bad customer service.
Some of my friends also check Facebook, LinkedIn, Twitter and the other social media services. But for me, a few quick searches on Google, Amazon and BBB usually tell me everything I need to know.
Nothing will kill a bad product faster than good advertising.