I use Eclipse Mars to compile a project. My understanding is that Eclipse is using it's own Java compiler (JDT) to compile Java code.
How can I find out the version of the Eclipse compiler so that I can tell other people what version I am using?
There isn't really a separate version for the ECJ compiler.
The standalone download of ECJ available on the Eclipse Project page is just numbered to match the Eclipse release (so 4.6 for the current Eclipse Neon release).
The compiler itself is included in the org.eclipse.jdt.core plugin which has its own version (currently 3.12.0)