Does cancer show strong symptoms before it spreads or are there no symptoms until it spreads?

Depends on cancer: Many have no symptoms till it is spread (metastasized)

Few if any symptoms: Most malignant tumors are asymptomatic until they spread. Breast cancer has no pain when detected and except for a mass or mammography finding there are no symptoms until say bone disease occurs and then pain may be noted. When colon cancer spreads to liver or lung there are no symptoms until the primary is detected and further work up detects metastasis. This is true of most tumors.

Detecting cancer: Many cancers are silent until they spread. Symptoms arise when they spread to compress nerves, blood supply or other vital functions. Some cancers produce hormones or mediators and can produce abnormal signs and symptoms drawing suspicion by means of abnormal dynamic metabolic function, and abnormal test results.