So, I'm sure you've heard of gatt.org being the fake website for the WTO. What I'd like to know is, when I'm on the Net, how do I discern the authenticity of the website? I was thinking something like Alexa, but where I don't have to pay to get extra details.
It doesn't seem like you are asking how to determine if a site is posing as the 'real' site, which would be phishing, but what you are really asking is how to validate the content of a site, which is more of a philosophical question than a technical one. How does one determine, for instance, that The Onion is not a real news site?
We can check the 'whois' information for the domain to see if it makes sense, but even then, there might not be enough data to go on. SSL certs might help to provide extra data, if one is used and not faked.
We can use Google searches to search for likely 'true' sites and compare, but that has its own obvious limitations.
How, then, can you determine that an otherwise legitimate site is offering valid and true information? You need to validate. Just ask wikipedia. They hope that with enough eyes and hands, the truth should bubble to the surface, but that is only marginally effective.
This goes back to the very first advice handed out when websites started to become popular: use your head and don't believe everything you hear.
apsillers comment of using extended validation certificate is correct extended validation gives you an extra boost in confidence you're at the right place. For sites that don't have that certificate though a few things you can do.
This is not a question answerable by technology.
What you're asking is how do I know if the site I want is the one I'm visiting. Which is a bit like saying: "how do I know if the phone number I'm dialing is the person I wanted to call" You're asking your computer to divine your unspoken mental intent rather than your instructions, which always leads to failure.
Instead, what you might do is look at what sites are popular. If I want to find out more about the Python programming language, type "python" into Google. Sites will be ranked according to relevance and popularity. Whichever one is most common will show up at the top.
This will still not always tell you what you want, because that's impossible. But it will tell you what other people want. And that's pretty close.