Check the About Us
If you've never heard of a particular streaming site, do a bit of sleuthing. All official sites have information pages that explain how their service works. Illicit sites are quite different.
Look for an "About Us" or "Help" link that further explains who made the site and how they back up their service. If the site says explicitly that it doesn't actually host any material and lists a disclaimer stating that it "merely" links to movies elsewhere on the Web, you can bet that it's not a legitimate business.
Read the fine print and you may see some eye-opening statements that would enrage any Hollywood investor. A lot of illegitimate sites actually boast about their illegal streams. Those are the kinds of sites you'll want to avoid.