How do you prevent/know your opponent might be cheating? [Serious]

Especially in long games (say 10 mins+) your opponent could be opening an incognito tab or another chess site.  He would play normally for the opening moves and in mid game starts using the engine.  By the time he is up 4 or 5 points on you, he will or attempt to trade away the queen and play normally without the engine.  Because by then you really have to be an idiot to lose while you are up 4 or 5 points on your opponent.  Of course you can't pull this off if you are like very low rated but if you are "decent" (whatever that means), the cheaters will argue that they just capitalized on your blunders instead of them using the engine (because you are a human and you are going to make mistakes and the engine is gonna play perfectly).  So what then?

Like how?  What did I do, the emotes are not serious but my argument is valid?  I think?

Hi, 

Cheating discussion please join the forum for that its not for general forums,

https://www.chess.com/club/cheating-forum

Also please report people if you are suspicious of anyone

https://support.chess.com/article/209-how-do-i-report-someone