How can I get a list of all configurations for a Gradle project?

With Gradle 5 it's very simple with the --info option. For example:

./gradlew projects --info

Now look in the Configure project section which lists all the configurations.

Add this to root project:

allprojects {
    repositories {
        // ....
    }

    task printConfigurations {
        doLast {task ->
            println "Project Name: $project.name configurations:"
            configurations.each {
                println "    $it.name"
            }
        }
    }
}

Then, for example:

$ ./gradlew -q :SubProjA:printConfigurations
Project Name: SubProjA configurations:
    -api
    -runtime
    annotationProcessor
    api
    apiDependenciesMetadata
    apiElements
    archives
    compile
    compileClasspath
    compileOnly
    compileOnlyDependenciesMetadata
    default
    implementation
    implementationDependenciesMetadata
    kotlinCompilerClasspath
    kotlinCompilerPluginClasspath
    kotlinKlibCommonizerClasspath
    kotlinNativeCompilerPluginClasspath
    kotlinScriptDef
    kotlinScriptDefExtensions
    runtime
    runtimeClasspath
    runtimeElements
    runtimeOnly
    runtimeOnlyDependenciesMetadata
    sourceArtifacts
    testAnnotationProcessor
    testApi
    testApiDependenciesMetadata
    testCompile
    testCompileClasspath
    testCompileOnly
    testCompileOnlyDependenciesMetadata
    testImplementation
    testImplementationDependenciesMetadata
    testKotlinScriptDef
    testKotlinScriptDefExtensions
    testRuntime
    testRuntimeClasspath
    testRuntimeOnly
    testRuntimeOnlyDependenciesMetadata

Try

gradle --console plain dependencies | fgrep ' - '

The dependencies task lists all configurations (along with their dependencies), and the fgrep will just show you the configuration names (along with a brief description of each). It's not great, but doesn't require you to put stuff in your build script.

Have you tried:

configurations.each { println it.name }

?