1. A method configured to be implemented on a control device connected with an inkjet printer, the inkjet printer comprising: an inkjet head configured to discharge ink droplets from nozzles formed in an ink discharging surface thereof; a head scanning unit configured to reciprocate the inkjet head relative to a recording sheet along a scanning direction parallel to the ink discharging surface; and a wave shape generating mechanism configured to deform the recording sheet in a predetermined wave shape that has tops of portions protruding in a first direction toward the ink discharging surface and bottoms of portions recessed in a second direction opposite to the first direction, the tops and the bottoms alternately arranged along the scanning direction, the method comprising steps of: acquiring gap information related to a gap between the ink discharging surface and each individual one of the tops and the bottoms on the recording sheet; and determining whether the gap information acquired for each individual one of the tops and the bottoms on the recording sheet is abnormal, based on a comparison between a deviation of the gap information from a reference value and a predetermined comparison value.