As every thing, it depends on the people and the companies. Some are serious, others not.
My advice is:
1. First, to check if the products or services promoted by you are useful, competitive and of good quality. To see if the products or services can be successful on the market.
2. Second, which are the side effects of the products or services? A product can look attractive at the begining, but (on the long term) can bring more pain than pleasure to the users. You have to be smart and ask to yourself logical and right questions.
3. Third, if the management of the company is good, if the trainings are professional, if the company is a creator or a copy of a copy, how big is the potential of incomes, etc.?
You can check on Google the name of Founder, CEO, leaders, etc. of the company (write with a "dot" separating the name ... e.g. "marc.planchette" ... myself :-) or "1000Club" ... my business :-)
Be careful with companies who ask you to invest more than USD 1.500...
And finally, ask to yourself if you will be able to growth and contribute by working with this company.
Hope my answer has given you a few directions to follow.