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.
compile(Deprecated) Compile time dependencies. Superseded by
implementation.
implementation extends compile Implementation only dependencies.
compileOnly Compile time only dependencies, not used at runtime.
compileClasspath extends compile, compileOnly, implementation Compile
classpath, used when compiling source. Used by task compileJava.
annotationProcessor Annotation processors used during compilation.
runtime(Deprecated) extends compile Runtime dependencies. Superseded
by runtimeOnly.
runtimeOnly Runtime only dependencies.
runtimeClasspath extends runtimeOnly, runtime, implementation Runtime
classpath contains elements of the implementation, as well as runtime
only elements.
testCompile(Deprecated) extends compile Additional dependencies for
compiling tests. Superseded by testImplementation.
testImplementation extends testCompile, implementation Implementation
only dependencies for tests.
testCompileOnly Additional dependencies only for compiling tests, not
used at runtime.
testCompileClasspath extends testCompile, testCompileOnly,
testImplementation Test compile classpath, used when compiling test
sources. Used by task compileTestJava.
testRuntime(Deprecated) extends runtime, testCompile Additional
dependencies for running tests only. Superseded by testRuntimeOnly.
testRuntimeOnly extends runtimeOnly Runtime only dependencies for
running tests.
testRuntimeClasspath extends testRuntimeOnly, testRuntime,
testImplementation Runtime classpath for running tests. Used by task
test.
archives Artifacts (e.g. jars) produced by this project. Used by task
uploadArchives.
default extends runtimeClasspath The default configuration used by a
project dependency on this project. Contains the artifacts and
dependencies required by this project at runtime.
A variation of @Mike Hanafey's answer using Gradle Kotlin DSL, and adding another task that prints only resolvable configurations (useful for passing to the --configuration parameter of dependencyInsight).