Grid-connected microgrids pose a risk of unsuccessful transition to islanded operation due to unintentional islanding. The success of this transition is influenced by different factors, such as the operating point, the controls, and the islanding detection scheme. This work proposes a methodology to access the probability of a successful transition of a microgrid from grid-connected to islanded mode. The methodology is based on Monte Carlo simulations using probabilistic curves for representing the operating point of the distributed energy resources, loads, and storage devices. The method is validated by performing an analysis of the islanding transition of a hybrid RE-storage-diesel microgrid, either employing a Battery Energy Storage System (BESS) or Flywheel Energy Storage (FES). The use of the proposed method allowed an assessment of the influence of the capacity of the storage units on the success of the islanding transition.