I wrote a while back about my recipe for Raspberry Cordial, a very simple mixture of raspberries, sugar and time. In it I mentioned several times how you can tell that the mixture is still fermenting when you see bubbles in the mixture when you stir it. I couldn't really show you what it looked like then because we were at the beginning of the process. I'd like to show you now, but unfortunately I'm not a great photographer. Here is what my somewhat fuzzy close-up of the top of the mixture looks like before stirring. Perhaps you can get a better idea of what I mean about the bubbles created by fermentation by looking at this photo of what the top of the broth looks like once you have finished stirring - distinctly frothy.
What, and miss the excitement of the Exploding Glass Bottle when I decide to bottle too early? Actually, that might be an option in the later stages but at the moment I know it's fermenting and I think the plastic wrap would just be too fiddly.
We went to a fruit winery in Wisconsin this weekend and they use a plastic sheet to monitor the CO2 production and fermentation. When the goop is fermenting, CO2 is given off and the sheet rises. When the fermentation is done, the sheet sinks back to the top of the vat.
I'm thinging cling wrap, myself.