If you search Google for [what is my user agent], Google will show you the answer. Just like they do for [what is my ip] or [speed test] and other computer related questions. Google is not linking you to a search result for the answer, well, you can go to those links, but there is no need to anymore.
Here is a screen shot:
It works both on desktop and mobile and was first spotted by @VorticonCmdr.
Forum discussion at Twitter.
