When you are looking for a work at home opportunity and come across a company looking for people it is important that you do your investigating before joining them. You want to make sure that the company is offering legit products and services and you also want to make sure that you are going to get paid. Sadly there is a history of some work at home companies not paying their workers. There are some brick and mortar companies that have did that but it is easier for a company that is out of someones house to be "Here today, Gone tomorrow". A company that has a physical location with employees that come in everyday is a bit more difficult to vanish overnight. It does happen but work at home companies are more likely to do that.
The first thing you want to do is look the company up on Google. See if there are any complaints about them. Just because a company has complaints does not mean they are a scam and not to go with them. Every company has someone who complains about them. You can't make everyone happy. However if there are consistent complaints about the same thing(s) then you may want to look into it further and you may not want to go with that company. Especially if it is in regards to not paying workers or having a scammy service.
The next thing you want to do is try and locate their business license. Go to the state that the business is located in Secretary of State's website. See if you can find the business and make sure that the are active and have not had their license revoked or anything. You may not be able to find the business. If you aren't that either means that they are using a different licensed name, their business is registered in another state, or they don't have a registered business. I will say that USUALLY they do have the business licensed under some name. Use your judgement with this one, if something doesn't seem right then don't pursue it.
I see people online always referring to the BBB. You are welcome to look the company up on it but I would not take what the BBB says very seriously. If you pay them several hundred dollars then you'll get an A+ rating. I recommend ripoffreport.com and complaintsboard.com those have real ratings that people can't pay to remove (or merge together).
Look at the documents they have you fill out. DO NOT disclose any personal information if you have any doubts about them. Don't be afraid to ask them questions. I ran into a incident where a work at home company sent me paperwork to fill out. They were all generic forms with the company name not on them. I couldn't find anything about them or find a business registered with their name. I was not comfortable disclosing my information to them when I can't even find a website.
The most important thing is DO NOT pay to work for them! Now there are companies out there that are legit that ask for a fee for a background check. This fee should not cost any more then $45. If you are asked to pay anymore then that for these "certification classes" then my advice is to not go with them. I hope this helps you out in your work at home job search.