If the gallbladder becomes infected it will make you feel ill with a temperature and increased white blood cells will be detected in the blood. Just like any other infection.
If the gallbladder becomes cancerous this can also be detected with blood test but not always. Certain groups are more prone to the condition as you probably know. It is also thought that pre-diabetic condition and inflammation of the gallbladder increase the risk. This UK website gives some good information: