Bistability plays important roles in biological systems, resulting in cellular memory, population diversity and differentiation. The existence of bistability requires positive feedback in regulatory circuits, within which ultrasensitive reactions must occur. In this talk I will illustrate how to map bistable parameter regimes experimentally using the open-loop approach. With synthetic designs in yeast cells, we were able to open transcriptional feedback loops and obtain deterministic open-loop functions. Combining these functions with kinetic measurements and reintroducing the measured noise, we were able to predict the transition rates for the feedback systems without further parameter tuning. Integrating experiments, modelling and simulation, this method helps identify the main determinants of cell fate transitions: bistability in the deterministic sense, noise, and transient kinetics, providing fundamental insights for future synthetic designs with engineered cell fate transitions.