A basic activity that listens to zen_mode state (Android's "do not disturb" volume/vibration muting feature) and dumps the current value to log. There are two ways included; the first is an observer and doesn't work in every case (I think there's a separate preference for priority messages), the second may is a manual check by hitting a button.

MainActivity.java

import android.database.ContentObserver;
import android.os.Bundle;
import android.os.Handler;
import android.provider.Settings;
import android.provider.Settings.Global;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import java.util.Locale;

import static android.provider.Settings.System.CONTENT_URI;


public class MainActivity extends AppCompatActivity {

  private static final String TAG = "ZenModeObserver";

  @Override
  protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    // Get a handle on the button for manually checking the zen_mode state
    Button manualCheck = (Button) findViewById(R.id.manual_check);

    manualCheck.setOnClickListener(new View.OnClickListener() {

      @Override
      public void onClick(View v) {

        try {

          logZenModeState();
        }
        catch (Settings.SettingNotFoundException e) {

          e.printStackTrace();
        }
      }
    });

    /*
      Create an instance of the ZenModeObserver below (it's actually just a global settings change observer)
     
      This listens for settings state changes and calls the logZenModeState() method
     */
    ZenModeObserver observer = new ZenModeObserver(new Handler());
    this.getApplicationContext().getContentResolver().registerContentObserver(CONTENT_URI, true, observer);
  }

  // Log the current zen_mode state
  private void logZenModeState() throws Settings.SettingNotFoundException {

    int zenModeValue = Global.getInt(getContentResolver(), "zen_mode");

    switch (zenModeValue) {

      case 0:
        Log.e(TAG, "DnD : OFF");
        break;
      case 1:
        Log.e(TAG, "DnD : ON - Priority Only");
        break;
      case 2:
        Log.e(TAG, "DnD : ON - Total Silence");
        break;
      case 3:
        Log.e(TAG, "DnD : ON - Alarms Only");
        break;
      default:
        Log.e(TAG, String.format(Locale.getDefault(), "DnD : Some other value in the future? Or obscure mfctr implementation? [%d]", zenModeValue));
    }
  }

  private class ZenModeObserver extends ContentObserver {

    ZenModeObserver(Handler handler) {

      super(handler);
    }

    @Override
    public boolean deliverSelfNotifications() {

      return super.deliverSelfNotifications();
    }

    @Override
    public void onChange(boolean selfChange) {

      super.onChange(selfChange);

      try {
        logZenModeState();
      }
      catch (Settings.SettingNotFoundException e) {
        e.printStackTrace();
      }
    }
  }
}

@kyleclegg not a clue, sorry.

If I were to look into DND detection these days, I'd dig into the ZenPolicy class docs, as it seems some of that is documented since API level 29; it didn't even exist when I first wrote this.

https://developer.android.com/reference/android/service/notification/ZenPolicy