How to check if an Activity is enabled?

I think the field ComponentInfo.enabled means the value set in the AndroidManifest.xml file. If not specified, the default will be true. So, in the following example:

<receiver android:name=".BroadcastReceiver1"
            android:enabled="false" />

<receiver android:name=".BroadcastReceiver2"
            android:enabled="true" />

<receiver android:name=".BroadcastReceiver3" />

the enabled field will be false in BroadcastReceiver1, true in BroadcastReceiver2 and true in BroadcastReceiver3, while pm.getComponentEnabledSetting(componentName) will return ENABLED or DISABLED if the value in AndroidManifest is overridden or DEFAULT if not overriden

Then, according to @Jared Rummler answer if pm.getComponentEnabledSetting(componentName) returns PackageManager.COMPONENT_ENABLED_STATE_DEFAULT the actual value will be the one set in ComponentInfo.enabled field which will be the same as the one set in AndroidManifest, while ComponentInfo.isEnabled() would depend on the whole application state

EDIT: To sum up:

public static boolean isComponentEnabled(PackageManager pm, String pkgName, String clsName) {
  ComponentName componentName = new ComponentName(pkgName, clsName);
  int componentEnabledSetting = pm.getComponentEnabledSetting(componentName);

  switch (componentEnabledSetting) {
    case PackageManager.COMPONENT_ENABLED_STATE_DISABLED:
      return false;
    case PackageManager.COMPONENT_ENABLED_STATE_ENABLED:
      return true;
    case PackageManager.COMPONENT_ENABLED_STATE_DEFAULT:
    default:
      // We need to get the application info to get the component's default state
      try {
        PackageInfo packageInfo = pm.getPackageInfo(pkgName, PackageManager.GET_ACTIVITIES
            | PackageManager.GET_RECEIVERS
            | PackageManager.GET_SERVICES
            | PackageManager.GET_PROVIDERS
            | PackageManager.GET_DISABLED_COMPONENTS);

        List<ComponentInfo> components = new ArrayList<>();
        if (packageInfo.activities != null) Collections.addAll(components, packageInfo.activities);
        if (packageInfo.services != null) Collections.addAll(components, packageInfo.services);
        if (packageInfo.providers != null) Collections.addAll(components, packageInfo.providers);

        for (ComponentInfo componentInfo : components) {
          if (componentInfo.name.equals(clsName)) {
            return componentInfo.enabled; //This is the default value (set in AndroidManifest.xml)
            //return componentInfo.isEnabled(); //Whole package dependant
          }
        }

        // the component is not declared in the AndroidManifest
        return false;
      } catch (PackageManager.NameNotFoundException e) {
        // the package isn't installed on the device
        return false;
      }
  }
}

If COMPONENT_ENABLED_STATE_DEFAULT is returned, how do I know if it's default as enabled or disabled?

You will need to load all the components using PackageManager and check the enabled state for the matching ComponentInfo. The following code should work:

public static boolean isComponentEnabled(PackageManager pm, String pkgName, String clsName) {
  ComponentName componentName = new ComponentName(pkgName, clsName);
  int componentEnabledSetting = pm.getComponentEnabledSetting(componentName);

  switch (componentEnabledSetting) {
    case PackageManager.COMPONENT_ENABLED_STATE_DISABLED:
      return false;
    case PackageManager.COMPONENT_ENABLED_STATE_ENABLED:
      return true;
    case PackageManager.COMPONENT_ENABLED_STATE_DEFAULT:
    default:
      // We need to get the application info to get the component's default state
      try {
        PackageInfo packageInfo = pm.getPackageInfo(pkgName, PackageManager.GET_ACTIVITIES
            | PackageManager.GET_RECEIVERS
            | PackageManager.GET_SERVICES
            | PackageManager.GET_PROVIDERS
            | PackageManager.GET_DISABLED_COMPONENTS);

        List<ComponentInfo> components = new ArrayList<>();
        if (packageInfo.activities != null) Collections.addAll(components, packageInfo.activities);
        if (packageInfo.services != null) Collections.addAll(components, packageInfo.services);
        if (packageInfo.providers != null) Collections.addAll(components, packageInfo.providers);

        for (ComponentInfo componentInfo : components) {
          if (componentInfo.name.equals(clsName)) {
            return componentInfo.isEnabled();
          }
        }

        // the component is not declared in the AndroidManifest
        return false;
      } catch (PackageManager.NameNotFoundException e) {
        // the package isn't installed on the device
        return false;
      }
  }
}

---

Testing the above code on my device:

System.out.println(isComponentEnabled(getPackageManager(),
    "com.android.systemui",
    "com.android.systemui.DessertCaseDream"));

System.out.println(isComponentEnabled(getPackageManager(),
    "com.android.settings",
    "com.android.settings.DevelopmentSettings"));

false

true