Common findings:
Leukocytosis
LFT abnormalities (obstructive picture)
Meta-analysis shows there is no history, physical exam, or lab test or combination thereof that allows rule-out or rule-in without imaging.
Leukocytosis
LFT abnormalities (obstructive picture)
Biliary ultrasound (preferred test; sensitivity 84%; specificity 99%)
Gallstones
Distinguish by characteristic "shadowing"
Better seen with patient in left lateral decub
GB wall thickening (>3mm)
May also be seen with pancreatitis, ascites, congestive heart failure, alcoholic hepatitis
Pericholecystic fluid
Sonographic Murphy's Sign (PPV 92%)
May be absent in patients with DM, gangrenous<unk> 
HIDA scan
Gold standard when other imaging modalities are equivocal
Other imaging
CT: there is a lack of evidence to support diagnostic accuracy
MRI: Accuracy similar to ultrasound
Gallstones
Distinguish by characteristic "shadowing"
Better seen with patient in left lateral decub
GB wall thickening (>3mm)
May also be seen with pancreatitis, ascites, congestive heart failure, alcoholic hepatitis
Pericholecystic fluid
Sonographic Murphy's Sign (PPV 92%)
May be absent in patients with DM, gangrenous<unk> 
Distinguish by characteristic "shadowing"
Better seen with patient in left lateral decub
May also be seen with pancreatitis, ascites, congestive heart failure, alcoholic hepatitis
May be absent in patients with DM, gangrenous<unk> 
Gold standard when other imaging modalities are equivocal
CT: there is a lack of evidence to support diagnostic accuracy
MRI: Accuracy similar to ultrasound
"Combining clinical, laboratory and imaging investigations is recommended, although the best combination is not yet known"
Acute calculous cholecystitis
Gallbladder disease
Acalculous cholecystitis
Symptomatic cholelithiasis 
Acute pancreatitis
Choledocholithiasis
Ascending cholangitis
Acute cholecystitis