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By unnatural, I don't mean ungrammatical, but something people wouldn't really say. For example, in many fictions, you find yourself in very weird situations and it's hard to know how a person would react and what they would say in such situations. Often, I feel people would stay silent and say irrational or dumb things, but that cannot really happen, but often when you make your character say something relevant, it often sounds very unnatural and sometimes even cringy. Let me give you an example:

Natalia: You turned me into a monster, how could you do that to me?

Robert: It was the only choice! You think we could have survived
otherwise? I made the call, because of that we're both alive. How
can't you see this!?

Natalia: You made the choice without letting me
decide my own fate. I cannot ever forgive you for this!

Robert: I don't care! Do as you want. If you want to die, go ahead, kill
yourself. It's as simple as it gets. No matter what you tell me, it's
not going to do any good. What's done is done!

Natalia: You pig!

As you can see, it kinda sounds awkward and ridiculous, but often it's not quite black and white, and it can be hard to tell especially if you've been writing a lot. So is there some kind of test or thought experiment you can use to make the determination that a dialogue is bad?

Trust your instinct. Period.

You are right--I zoned out at 'You turned me into a monster.' Who says that? I mean, I don't know whether to cringe or LOL. I didn't read further, but forcing myself to do so--No. Just no.

Try this:

Natalia: "F*ck you."

Robert: "I made the call. We're alive. End of story."

Natalia scoffs in disgust. "Alive at what cost," she says under her breath.

Robert: "If you want to die, go ahead. Otherwise, shut up."

You are trying to explain so much in your dialog that I think we should coin the term 'diodump.' Trust your reader to get the emotional message without the technical details. Monster? What? Not needed.

ETA in response to comments:

Natalia: "F*ck you." She felt like a monster. Like he'd violated her--even though she'd signed the consent form, she never thought he'd actually give her the nano-bot injection.

Robert: "I made the call. We're alive. End of story."

Natalia scoffs in disgust. "Alive at what cost," she says under her breath. The knowledge that she'd spend the rest of her life with those things inside her, coursing through her veins, it made her ill.

Robert: "If you want to die, go ahead. Terminate the bots. Otherwise, shut up."

Sadly I don't think there is an ISO standard test or something this can be run against, but like most parts of writing we can apply various tools to help us evaluate things.

Examples of some of the evaluation steps I run stuff through:
1. Does it fit with the character's other dialog, and does the pacing and tone match the scene? "I cannot..." while in a hurry might not flow as well as "I can't", unless someone is super formal by nature.

2. 
   

   Does it sound good?

   
   
   
   
   
   • Read it to yourself, have someone else read it to you, record and play it back, and ask yourself "Does this work?"
   
   
   • I personally find piping my work through text-to-speech software surprisingly handy for this.
   
   
   
   


3. 
   

   Does it get good feedback?

   
   
   
   
   
   • Beta readers are your friends [but sometimes you shouldn't use your friends for beta readers - Joining a writing circle or similar may be useful.] - Do other readers find it stands out in odd ways?
   
   
   
   


4. 
   

   Can better words be found?

   
   
   
   
   
   • If you're unsure about if you like a group of words, there is always the option to set the current ones aside and just rewrite the section for the sake of deciding if you like one better than the other.
   
   
   
   

Also keep in mind the point that at times much more can be said with silence than with a thousand words.

Read it aloud. Flaws are often more apparent when heard - particularly in matters of flow and pacing.

Would Natalia say that? How would Robert answer or would he even bother?

Get inside your character’s head. What would you be feeling, thinking and eventually saying? Would you say anything at all?

Perhaps a glare of astonished hatred would serve and say more with silence than words. Have her think livid thoughts, feel the outrage and loss of control. Be her - then slip inside him and respond.

Would Robert just walk away, his task complete? Might he simply look at her with fond regret, seeing the alterations of which she is as yet unaware.

She woke - almost surprised to as the last thing she remembered was
getting hit, falling. Robert was there, had called for help. Wait -
that blaster hit should have killed her - was killing her. Had it?

She saw Robert, his back to her. He closed his surgical kit and
disposed of four syringes. Wait - red syringe meant bots. God, not
that. She clenched the side of the bed, not noticing the damage to the
rail.

Robert turned, seeing her glare. He noticed the damaged rail and knew
it was a complete success. It would take time for the bots to complete
their work and the prosthetic devices would work as well as her
natural arm had - maybe better. The eye was not a good match to her
natural colour, but she would be operational in a matter of days.

He should avoid her until she adapted to her new life, she would thank
him later. No choice, no time to waste. If she didn’t thank him, at
least she was alive to hate him.