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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What you can do is play two or three games with them simultaneously. Work out their average time they take to make a move. They could cheat in the first game because they could take any length of time, but after their first go, you should be able to tell if you are both playing at the same time. If they don't play their moves regularly, but always beat you on the first game and not the second, then it might indicate that they cheat. Or if they take unusually longer on their second game, and suddenly get an unusual word, that's also a clue.
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.