How does a cell decide to do fermentation versus the krebs cycle?

The cell decides to ferment instead of the krebs cycle when there is no oxygen in the cell cytosol.

Explanation:

Fermentation occurs in the absence of oxygen in the eukaryotic and prokaryotic cell cytosol. In this situation glucose is degraded into simpler substances such as lactic acid (lactic fermentation) and ethyl alcohol (alcoholic fermentation). In both lactic and alcoholic fermentation there is a balance of only 2 ATP molecules and, in both processes, they start with pyruvic acid obtained from glycolysis, as described in aerobic respiration.

In a nutshell, we can say that fermentation is an anaerobic process of obtaining energy, ie occurs when there is no oxygen available in the medium. The fermentation may be an alcoholic fermentation or a lactic fermentation.

The initial stage of fermentation is the same as cellular respiration: glycolysis. The energy yield of fermentation is lower, only of the 2 ATP that are produced in glycolysis. But it is a faster process than aerobic respiration.