I would deem cheating in Words with Friends to use a dictionary or site that generates words in a manner of best score.

I have had opponents play 100 + point words that I didn't even know existed.

Is there anyway without being with them / watching them play to see if my opponent is 'cheating' ?
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If it it someone you know, the best thing to do is ask them if they think it's fair to use the cheater apps or sites. My good friend said she thought that was quite legit and I told her I prefer not to play like that. So now we don't.
It is indeed hard to figure out if your friend is using an online cheat tools or dictionary when playing Words with Friends, since you cannot see them in person. But there are other signs which could help you to figure out whether they are cheating or not such as your friend tend to use difficult or unknown words consistently and they are getting huge points in every move. If you want to know more [How to Catch a Friend Using Words with Friends Cheat](http://gameolosophy.com/games/how-to-catch-a-friend-using-words-with-friends-cheat/) you can read this article for additional information.