How can I know that my WebView is loaded 100%?

The best way to detect if a page has rendered is to use the onPageCommitVisible callback, available from API 23. onPageLoadFinished is not suitable, since it's delivered too soon (when the HTML is processed, but not yet rendered).

webview.setWebViewClient(new WebViewClient() {

    @Override
     public void onPageCommitVisible (WebView view, 
            String url)
    }
}

As said here: How to listen for a WebView finishing loading a URL?

boolean loadingFinished = true;
boolean redirect = false;

mWebView.setWebViewClient(new WebViewClient() {

   @Override
   public boolean shouldOverrideUrlLoading(WebView view, String urlNewString) {
       if (!loadingFinished) {
          redirect = true;
       }

   loadingFinished = false;
   view.loadUrl(urlNewString);
   return true;
   }

   @Override
   public void onPageStarted(WebView view, String url, Bitmap facIcon) {
        loadingFinished = false;
        //SHOW LOADING IF IT ISNT ALREADY VISIBLE  
    }

   @Override
   public void onPageFinished(WebView view, String url) {
       if(!redirect){
          loadingFinished = true;
       }

       if(loadingFinished && !redirect){
         //HIDE LOADING IT HAS FINISHED
       } else{
          redirect = false; 
       }

    }
});

webView.setWebChromeClient(new WebChromeClient() {
            public void onProgressChanged(WebView view, int progress) {
                if (progress == 100) {
                    //do your task

                }
            }
        });