If we were a synth like in Fallout would we know? Have you ever cut open your gut and checked?

That's a sneaky switch from we to you. Meaning: Collectively, humanity definitely have done and proven the latter, more so than necessary.

And if we were to dive deeper into the Philip K. Dick style fantasy realm you're hinting at: What's stopping an android's sensory system from being augmented to perceive only what they're meant to see? Heck - Why wouldn't the 'parts' be made in a way where they're indistinguishable by us either way?

Once you go down that line of questioning, it's hard to win!

The X-rays make it clear enough.

not intentionally.