Rowan,
I understand what you're saying. One thing I did notice with those powerful solvents was (if submerged long enough) that the chain links would be stiff. That's when I knew it was completely degreased. I couldn't prove the lube was replacing what the solvents had taken away and it was a concern to me.

In practice, I have used the clip on chain cleaner and then removed the chain. It looked pretty clean. I smeared shampoo on it and rinsed it off. I was surprised at the amount of additional grease that came out of the chain. I did the same "test" with WD40, Gunk chain lube/cleaner, and mineral spirits. Same results...more grease came out.

If you care to duplicate what I've done, use a piece of plastic sheet to lay the chain on as you hose it down. My bathtub was a bear to scrub to get those grease stains out.