1. A method of identifying potential locations of phantom images from a mask layout during simulation based checking, the method comprising: performing one of sizing up features in the mask layout by a predetermined amount and computing rings for the features in the mask layout; counting overlap areas caused by one of the sizing up and the computing rings; if a total weight of an overlap area exceeds a trigger weight, then adding an evaluation point to the overlap area to identify that location as a potential location of a phantom image; and performing the simulation based checking of the mask layout by simulating a wafer image at the potential location and comparing a result of the simulating to the location of the added evaluation point on the mask layout.