How can I tell if my opponent is cheating in Words with Friends?
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.
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.
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.