How do you know that the information you read on any website is reliable?

Credible sites for facts are governmental, for instance fbi.gov, educational - MIT, NASA or other known contributors such as McGraw hill. Such sites are thought trustworthy...

Other site like wikipedia.org (or this one...) where anyone can contribute should not be trusted unless the fact is backed up by another trustworthy source.

If Hillbilly Mike (got no proper education) starts a website about quantum physics....

So to sum up:
Look at the domain name, .org, .gov, specific country domain name (.se, .co.uk, .fr etc)
Is the site layout professional?
Is it a known site?
Do they have sources from where they got their facts? (book, publication, other site)