Hello~Yes, it will but if it is a very small tumor, such a small acoustic neuroma, then it possibly will not catch it, but those are fairly rare. An MRI is actually the gold standard for locating any type of brain tumor, but since they can be expensive, the CT scan is usually the test of choice as it is usually very accurate.
Yes, a CT scan or MRI would almost always show a brain tumor. Are you afraid you have one? Do you have any symptoms and is your doctor recommending this imaging?