How do you know if a given web site uses QUIC/GQUIC? It is a great question, and can be easily answered if you are using Google Chrome (which, by now you should be!).
First, you can check to make sure QUIC is enabled in your Chrome browser. Do this by navigating here:
You should see that QUIC is enabled:
Once enabled, you can see if a given site supports QUIC (all the google sites do, of course) by opening a new tab, then hitting Shift-CTRL-I. This will open the Chrome devloper tab either at the bottom or on the right of your page (I like mine at the bottom):
Now that is done, make sure your look is the same as what I show above, (select the network tab), and disable the cache, and you may need to drag some panes and such.
Next open a page, like youtube.com in that tab or just do a Google Search:
To see the protocol column, right click on the Name row, and add protocol if you don't already have it.
You will see in that protocol column that QUIC is clearly being used.
If I open www.cellstream.com in that tab, I clearly see it is not:
Hope that helps you learn just a little more about QUIC.