You can access the tool by going to validator.ampproject.org or download the chrome extension as well. The chrome extension is a bit cooler because it quickly gives you a link to your AMP URL, and then when it takes you there, it quickly shows if you have validate AMP markup.
The web version requires you to copy and paste the AMP page URL and/or the source code into the AMP validator and it checks right there. Both tools are much easier than using the Chrome debugger tool.
Here is a screen shot of my AMP page validated by the web UI:
Here is the chrome extension where the blue means the web URL has an AMP URL reference and when you click it, it opens the AMP URL and if it is green, it means you are valid.
The chrome extension icon in red would mean it is not valid and here is what it looks like on the web UI when it is not valid:
Forum discussion at Twitter.