How do I know which Beta Version of OS X I'm running?
When I check About this Mac it shows me that the name of the OS version is OS X El Capitan, the version if 10.11 and a beta, and it shows me the build number, but how would I find out which specific beta release this is?
IE, how would I find out if build 15A235d as in the picture below corresponds with OS X El Capitan Developer Beta 4, 5, or 6?
Is there a way to find it out within the OS? Is there some kind of public database online like everymac.com where every build number is matched up with every name it was given?
Login to developer.apple.com with your developer account, you should see in the download section the current release build on the download page for OS X.
This is to answer how to find the build as per OP.