Ideally you would need a bottle of Chanel Allure Homme, the original clear bottle, of the same size, to use as a guide. Weigh it, weigh the opaque bottle then subtract and you are left with the weight of your bottle you are wanting to sell then you work the weight out as a percentage. I would also suggest removing the caps before you do this as the Allure Sport and Edition Blanche caps are heavier than the original one. I.e. if you had a half full 50ml Chanel Allure and your Allure sport was 25% heavier then you could be almost certain that there is 25% more juice.

This would be my only suggestion. I did the same with Creed bottles to find out how much Himalaya I had left.