About opaque bottles: Is the bottle metal? It might be a different temperature above the fluid level. (For what it's worth, lips are very sensitive when it comes to discerning fine temperature differences.)
If you have access to full bottles, you can estimate the weight of a full bottle compared to the one you are selling. You can shake the full and the partial ones and get a feel for the difference in sound.
Sorry to read that you have been sick-- and that the entire wardrobe has to go!