Because Apple's iOS is very secure, it's difficult to determine if Robo Shield is working. If you are not getting any spam calls there is no way to determine if the application is working.
However, if you get just a few spam calls, you will not receive the call but instead, these calls will be routed to your voicemail. Check your voicemail for spam calls. If you find them, it means that Robo Shield is working.
If Robo Shield is acting as a free application, scam calls will be identified when your phone is ringing, which means that Robo Shield is working but is not using the premium features. If this is happening, please review the following article.