We do not need to specify the versions and other parameters of Maven dependencies and plugins we use. We can do it only once in parent POM or may be in some other way and inherit that properties in our children POMs.
Unfortunately for many times I have met the projects where I cannot find the real parameters of dependencies and plugins that easily. I.e. the versions of dependencies are empty, but in parent POM there is no information about this.
So my questions are:
What are all the possible ways to specify the parameters of dependency and plugin in Maven? I.e. where can I look for version number if it is not present neither in plugin in child POM nor in plugin management in parent pom?
Is there any Maven command to find out where Maven takes these parameters from? For example, I may use the dependency:tree command. It will print a list of dependencies which will help me to understand the version dependency I am stuck about. But this is a hack, and it cannot help me with plugins. May be there is something better?
Before executing any command, Maven builds the "effective POM", see this stackoverflow answer for more details. You may view the effective POM for any project by running mvn help:effective-pom. Typically when I run this command I redirect the output into a file so I can view and search it in a text editor.
You asked about specific parts of the POM, dependencies and plugins. You're on the right track, the maven-dependency-plugin helps. The goals I use most often are dependency:tree, dependency:resolve, and dependency:resolve-plugins. The first two assist with project dependencies, the last one with plugins. Add the -U option to force Maven to update dependencies regardless of the update policies/repository metadata values.
It is also helpful when troubleshooting to start with an empty local repository.
Last but certainly not least, Maven will give warnings when a build uses a plugin without a specific version. ALWAYS specify an explicit version to fix the warning and avoid issues like this one.