Previous research has shown that visual search performance is modulated by the current contents in visual working memory (VWM), even when the contents of VWM are irrelevant to the search task. For example, visual search is faster when the target - rather than a distractor - is surrounded by a shape currently held in VWM. This study uses the modulation of visual search by VWM to investigate properties of VWM. Participants were asked to remember the color or the shape of novel polygons whose "goodness" of figure varied according to Garner's (1962) rotation and reflection transformation principle. During the memory retention interval, participants searched for a tilted line among vertical lines embedded inside colored polygons. Search was faster when the target - rather than a distractor - was enclosed by the remembered polygons. The congruity effect diminished with increasing memory load and decreasing figure goodness. We conclude that congruity effects in visual search can indirectly assess VWM representation strength.