You should create and set a WebViewClient and override its shouldOverrideUrlLoading() method. That way you get the chance to intercept any links clicked (or redirections, &c).
For example:
webView.setWebViewClient(new WebViewClient()
{
@Override
public boolean shouldOverrideUrlLoading(WebView view, String url)
{
if (url.endsWith(".json"))
{
Toast.makeText(MainActivity.this, "json found", Toast.LENGTH_SHORT).show();
return true;
}
return false;
}
});