How to know my app is uninstalled from the device...?

My proposal will be the following. You can intercept the intent for your application uninstall. Simply put the following code in your manifest file:

<uses-permission android:name="android.permission.INSTALL_PACKAGES" />
<uses-permission android:name="android.permission.DELETE_PACKAGES" />
<uses-permission android:name="android.permission.CLEAR_APP_CACHE" />
<uses-permission android:name="android.permission.READ_PHONE_STATE" />
<uses-permission android:name="android.permission.CLEAR_APP_USER_DATA" />

<application
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name" >
    <activity
        android:name=".UninstallIntentActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
        <intent-filter>
            <action android:name="android.intent.action.VIEW" />
            <action android:name="android.intent.action.DELETE" />
            <category android:name="android.intent.category.DEFAULT" />
            <data android:scheme="package" android:pathPattern="com.testpack.yourapp" />
        </intent-filter>



    </activity>

</application>

After this you can somehow process that your application is going to be deleted (for instance, send you a email), and call the package manager uninstaller.

You cannot tell when someone uninstalls your app unless you keep track on the statistics in android market. And unless you have only a few installs, that may prove difficult