You’re trying to find a gotcha where there is none. I’m telling you that your question is incoherent.

The sex of an organism is defined as the size of the gametes it is organized around producing. That’s it. The secondary structures just tell you what that’s likely to be, because they’re correlated with it.

You’re trying to posit a “spherical cow”, a theoretical construct that doesn’t exist. A body won’t just “not have gonads”. You’re talking about magically poofing someone’s gonads out of existence. It’s the same as asking “Oh yeah, well if I was a rectangle, what sex would I be?”

I’m explaining the more reasonable and coherent case of “Assume you can’t examine the gonads of a body. How can you fairly reliably determine their sex by looking at secondary structures”? Note that it’s “fairly reliably” here because it’s entirely the gonads that define sex (pre-emptively, yes it’s gamete size, no I’m not changing the definition, but gonads are what produce gametes, stop trying to misread plain language for gotchas). If you restrict yourself from looking at gonads then you’re limiting yourself to correlates