Click on the Apple symbol in the upper left corner of screen > About this Mac > System report > scroll down to Software (left side) You should see OS X 10.10 (14A389) if you are running the GM.
Click on the Apple menu, choose About This Mac. Click on Version under OS X Yosemite, which should display the build number. The public release version is build 14A389.
Thanks folks. I was looking in the right place but wasn't clicking version. I was wondering why the build wasn't there, it just said 10.10, but I indeed had to click it.