What are signs of cancer in the back?

Weakness: I assume you mean spine tumors, not skin, muscle or CSF cancer. Cancer of the spine can often come from other tumors, like kidney or prostate. Workup frequently involves imaging the spine to do staging. Can also have nerve cell tumor or other tissues. Signs may be pain from vertebrae collapse and nerve injury and weakness, loss of bowel, bladder control (emergency) from spinal cord compression.