Crystallization of a liquid below liquidus temperature is a complex process due to simultaneous nucleation and growth of crystals. Nucleation is the crucial initial step of the crystallization process, and affects the glass-forming ability, especially when there is a large overlap between nucleation and crystal growth versus temperature curves. From the temperature-time-transformation (TTT) diagram, one can estimate the critical cooling rate, q*(c), of glass-formation, however this is time-consuming. In this paper, we establish a simple approach to determine the q*(c) using calorimetric and viscometric data. Based on the classical nucleation theory, the correlation between the crystallization onset temperature and cooling rate is described by combining two temperature-dependent functions. The new approach is applicable to a wide range of glass-forming systems. This work also gives insight into heterogeneous nucleation and glass formation kinetics.