Show Admob Test Ads

Step 1: Manifest

<manifest>
    <application>
        <!-- Sample AdMob app ID: ca-app-pub-3940256099942544~3347511713 -->
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713"/>
    </application>
</manifest>

Step 2: Use test ids:

App Open ca-app-pub-3940256099942544/3419835294
Banner ca-app-pub-3940256099942544/6300978111
Interstitial ca-app-pub-3940256099942544/1033173712
Interstitial Video ca-app-pub-3940256099942544/8691691433
Rewarded ca-app-pub-3940256099942544/5224354917
Rewarded Interstitial ca-app-pub-3940256099942544/5354046379
Native Advanced ca-app-pub-3940256099942544/2247696110
Native Advanced Video ca-app-pub-3940256099942544/1044960115

Step 3: Setup app-ads.txt file that includes this line:

            google.com, pub-3940256099942544, DIRECT, f08c47fec0942fa0

Source: https://developers.google.com/admob/android/test-ads