There is a small but important bug with the Google Structured Data Testing Tool where if you try to use it, there is no button to click to see the structured data.

Here is a screen shot of what it looks like:

Notice, the “preview” text is not a button and clicking on it does nothing.

It should look this this (from 2012:

I assume this is a very easy bug for Google to fix and they will resolve it shortly after they are notified.

Rindo Vincent first noticed this and informed me of this.

Forum discussion at Twitter.