If you want to know that someone using whatsapp plus is not easy and you cannot be fully sure about this. But you can check some options to know this. All you know that whatsapp plus is a modified application for android. It has some extra features that you will not get in the official whatsapp apk.
To know if someone is using whatsapp plus then you have to look on some features that you are not getting from official one users.
Last Seen: Normally when you are going to see any contacts on whatsapp can see when the user was online last time. There is an option that you can hide this option in whatsapp plus. If you are not getting this information then you can think that the user is using the modified app of whatsapp.
Online Status: Whatsapp+ has a feature that it can hide the online status. If you are talking or chatting in whatsapp and not able to see the status ever, then it is a sign that person using the mod application.
Double Blue Check: When you send a message to someone via whatsapp and the user read the message it shows the blue check mark and it is clear that the message has been seen. But if someone using the mod whatsapp then there is a possibilities that you will not be able to get that information.
Emoticons: In official whatsapp there are built in emoticons available. But if you are getting some additional emoticons that is not available in your device there is something to think about. So, the main point is there could be more emoticons.
Typing notification: When someone typing on whatsapp you can see a notification on the screen that the user is typing. In whatsapp plus user can hide this option and you will not be able to see the typing.. text on the screen.
Final Words: Finally it is clear that if someone follow the above things then can assume that the user using the whatsapp plus apk. Also you can ask the user if found that kind of things while chatting with someone.