How can I find the version of the Eclipse Java compiler (JDT)?

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)

You need to first go into Java EE Perspective. For this go to the ribbon and then:
Window > Perspective > Open Perspective > Other > Java EE.

NB: If you don’t see this then it means you don’t have the Java EE perspective installed: See how to download it here:

How to add Java EE perspective to Eclipse

After that, open the Package Explorer. Then right click on your Java Project and select Properties. In the properties window, there should be an option for “Project Facets”. This page lists the versions of all the Facets in your compiler. You can then see the version number for the Java Compiler as well.

If you need to change this, click Modify Project. Then, click the facet you want to change it too. Do this by selecting the version from the drop-down menu next to the Facet’s name (Java Compiler). Click finish and then click OK.

Hope this helps!