How to get the build variant at runtime in Android Studio?

Look at the generated BuildConfig class.

public final class BuildConfig {
  public static final boolean DEBUG = Boolean.parseBoolean("true");
  public static final String APPLICATION_ID = "com.example.app";
  public static final String BUILD_TYPE = "debug";
  public static final String FLAVOR = "";
  public static final int VERSION_CODE = 1;
  public static final String VERSION_NAME = "";
}