How can I check which Gradle Android plugin version is used in my project?

Newer versions of Gradle (3.0+, and possibly earlier) support a built-in buildEnvironment task that shows the dependency graph for buildscript.

./gradlew buildEnvironment

https://docs.gradle.org/current/userguide/tutorial_gradle_command_line.html#sec:listing_buildscript_dependencies

Showing the resolved dependencies for the build script class path isn't a built-in operation as for other configurations (which can be inspected with gradle dependencies). However, you can write a task to do so. For example:

task showClasspath  {
    doLast {
        buildscript.configurations.classpath.each { println it.name }
    }
}

A variation that just shows the Android plugin version:

task showVersion {
    doLast {
        println buildscript.configurations.classpath.resolvedConfiguration.firstLevelModuleDependencies.moduleVersion
    }
}