1. A method of checking a mask layout comprising a set of main features and a set of sub-resolution assist features (SRAFs), the method comprising: dividing each edge of each main feature into a set of sections; forming a set of segments by searching away from each one of the set of sections over a distance to determine if any portion of a feature is located within the distance; flagging each unassisted segment that does not include any portion of a feature located within the distance; and storing segment data comprising the set of flagged segments for each of the set of main features.