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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It is totally impossible to be able to tell if someone is 'cheating' unless you go to their house and check their phone browser history for any of those word helping websites. Other than that, what's to say they aren't just good with words? 

It is also important to note that the complexity of the word or length mean nothing in comparison to playing the game well. Knowing words helps, but dropping those big point letters on the triple letters and dropping those big point words on the triple words is what nets you the big points. I bet you could score 100 point with the word Zoo if you placed it well enough. Words with friends is a lot of strategy not just word knowledge.

A couple tips to beat them at their own game.

1. **Take longshots**: What's the worst that can happen if you try to put in a word that's not real? It's not like someone is looking over your shoulder saying "Haha you loser, you don't even know what a word is.". Try to land those high letters on those triples.
2. **Don't set *them* up:** If you're making a word, be very concious of where *they* will be able to go next round with your word. Did you just open up a path to a double word? A triple word? Make sure you're leaving them with basically nothing awesome.
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.