Had l spine xray and CT scan for kidney stones, all clear. Low back pain. Would l spine xray have shown lower back tumor? No stones, uti, etc.

No. An x-ray of the lumbar spine would predominantly show only bone. In general, an x-ray is not a very sensitive test to detect the specific cause of back pain in most patients. Consider requesting an MRI which shows the bone, but also shows the nerves, discs, and other soft tissues of the back and is therefore much more sensitive than an x-ray.
No. X-ray does not show soft tissue well. So, if tumor, ct or MRI would be needed to show it. You mention that you have had a ct for stones. Usually this is abdominal ct and pelvic ct. If so, it would show low back. Your doc can tell you how much showed on ct and can discuss whether there is a reason for you to worry about a tumor.
Not sensitive test. Xray will show large obvious bony masses but this is not asensitive way to evaluate for cancer. Consider a pet scan or MRI depending on symptoms. But before you scare yourself too much, why are you questioning if you have canser. Do you have weight loss, fevers, chills, night sweats, or other symptoms that make yuo think you have cancer? Because of your questions, you need to see your pcp.
Possibly. It might, but some tumors are not visible with a non-contrast ct scan.