Never pay to join a website
Any legitimate website will never ask for a payment to join. You shouldn't have to pay money to make money.
Look for proof of payment
Many websites that are legitimate will post proof of payment. However, just because a website doesn't list this doesn't necessarily mean they are a scam.
Do an internet search
Usually if you search the website or companies name, it will provide information about them. It's fairly easy to read reviews of websites and companies. You can also read what people are saying, to find out if it is a scam or not.
With so many making money opportunities available on the internet today, it's easy to get overwhelmed and even confused. It's best to read reviews from people who have tried to work for websites and companies. The reviews can also tell you whether or not they are worth your time. Just be careful of people who are trying to get referrals and make the site look and sound really good.
Look the BBB sign, but not everywhere has this
You can go to www.bbb.org if you have a business in a particular location you want to look up.
You can try Scam Adviser but don't take their word
I've used scam adviser myself. However, some websites such as Swagbucks (one of the most popular websites) have a low rating. I myself have been paid by Swagbucks many times so i don't know why they have such a low rating. It still doesn't hurt to look at scam adviser because you can also scroll down and see people's comments.
Search, post, and ask about it in work at home forums and similar places.
There are actually quite a few different forums and similar places on the internet that you can gather and search information.
Be aware of "get rick schemes"
Be aware of "Pyramid scemes"
Your best bet is to do a search on Google, and Never pay to join a website.