Ultrasound is considered the gold standard for detecting gallstones. 
Some radiopaque gallstones may be seen on plain film.
On CT scan images calcified gallbladder stones are hyperattenuating to bile, making them the only type to be clearly visualized on CT scan images. Pure cholesterol stones are hypoattenuating to bile, and other gallstones are isodense to bile and these may not be clearly identified on CT scan.
On MR images gallstones seen as signal void or low signal outlined by markedly hyperintense bile within the gallbladder.