Normally what I would do, is use Google to search for reviews on that particular website for example. But be careful when you’re reading reviews because sometimes some of them are written by people who are affiliates and who want to sell the website. Another thing is, if they ask you to pay money to be a member, it’s definitely not legit.
I use a variety of websites and word of mouth from people I have worked at home with along with ripoffreport.com, BBB.org, and google. It can get difficult researching work at home (that’s why it usually takes me so long) especially because along with the paid affiliates you also get the people who are angry or competitors that go around bad mouthing actual legitimate companies. You would be amazed at how some companies choose to treat their competitors, they are usually not good companies to work for. While researching you have to take into account that ever single company out there will have something bad said about them and use your gut and brain to decide whether or not to go with them. When in doubt go somewhere else. A good rule of thumb though is if most or even half of the reviews/comments are bad then turn and run the other way.