Formation of a microlayer in nucleate boiling results in substantial changes to the heat-transfer dynamics of the process, and establishing a criterion to distinguish between the microlayer and contact-line regimes encountered is necessary to obtain a quantitative description of the nucleate-boiling phenomenon. In this work, we describe microlayer formation as a dewetting transition in the presence of phase change. We derive a specific criterion for modelling the transition based on a synthesis of existing theoretical, experimental and numerical data. As a result, a new formulation of the transition criterion is developed, and applied to (i) reference data from a dewetting experiment of a volatile liquid, and (ii) results derived from a high-resolution direct numerical simulation of nucleate boiling. In both cases, very good agreement is found for Jakob numbers 75, a range which covers many important boiling situations.