Does an x-ray detect a tumor/cancer in the neck?

Not exactly: X-rays help you detect abnormalities such as a mass that is not normal but they do not tell you what that mass is. In order to figure this out you need to have a biopsy. The tissue obtained at the time of a biopsy is then reviewed by a pathologist who can help identify whether the mass is consistent with cancer. Other imaging tests like ct scans or MRI can be more helpful in delineating anatomy.

Maybe: If it's just a plain / regular x-ray study, many cancers in the neck will not be picked up using this method. However, a cat scan (ct), MRI or ultrasound study may be more likely to detect a cancer in the neck.