If you know how the final page is, you can analyze it in some of the delegates methods of the WebView. For example implemeting something like this in the class that is the delegate of the WebView:
- (void)webViewDidFinishLoad:(UIWebView *)webView{
NSString *myText = [webView stringByEvaluatingJavaScriptFromString:@"document.documentElement.textContent"];
if ([myText rangeOfString:@"order complete"].location != NSNotFound) {
//Your logic stuff
}
}