Had CT scan to look for kidney stones, do they check gall bladder & appendix also, or only look for kidney stones?

Most of them do. Most of the times, the radiology tech covers both abdomen and pelvis. Some radiologists are thorough and read the entire abdomen including liver and gall bladder.Some are limited.
Depends. The scan images will most likely include images of other organs/structures around the kidneys, but the extent to which it does depends on whether they scanned your whole abdomen/pelvis or just focused on the kidneys. To some extent it depends on whether they used contrast or not as well. The radiologist will generally comment on all organs seen in the scan, so ask your doctor what it says.
Not usually. Ct for kidney stones is usually done without injection of intravenous contrast. Unless gall bladder shows calcified stones ultrasound, hida nuclear scan, and ct with contrast are better to evaluate gb. Appendix may be visualized. Usually intravenous contrast makes it easier to visualize the appendix and associated inflammation with certainty. Ultasound evaluates gb for stones and inflammation.