How do you judge the trustworthiness of content on the internet?

My sixth-graders are taking a class called "Digital Citizenship" where they are learning about trusted online sources. A recent post makes the point that as the amount of content available on the Internet has grown, the trustworthiness of content on the Internet has decreased. Trying to explain to my kids that everytihng online is not true,  it leads us to the question ... How do you judge the trustworthiness of content on the internet?