Weigh the whole apparatus before you start the fermentation, keep in a place away from where too much evaporation will happen, then weigh it again afterwards, and most of the mass difference will be CO2 lost.
the aparatus includes the beaker full of limewater, so ther will be no mass loss. would the best way to do it be weigh beaker of limewater before and after the fermentation, and also mabye weigh conical flask before and after and get an average mass loss of the 2?
no - because not all of the CO2 will react with the limewater or some of the CO2 will be atmospheric and not from the experiment. So you would weigh them separately before and after, the mass change of the limewater isn't a really accurate indicator.