Looking at the "effective POM" will probably help too. For instance, if you wanted to know what the path is for ${project.build.sourceDirectory}
you would find the related XML in the effective POM, such as:
<project>
<build>
<sourceDirectory>/my/path</sourceDirectory>
Also helpful - you can do a real time evaluation of properties via the command line execution of mvn help:evaluate
while in the same dir as the POM.