The problem with cancer is that it is possible for one cell to grow into a tumor, but there's no way to detect one cancer cell. After cancer surgery, it's usually not possible to detect any remaining cancer using the tests that are available, but that doesn't mean that there aren't still a few cancer cells lurking there. So they do chemo and/or radiation hoping to kill any remaining cancer cells, even though they don't know whether or not there are actually any still there.

So there's no way to tell whether treatment worked or not. If the cancer doesn't come back, either the treatment worked, or it wasn't necessary in the first place. You only know it didn't work if the cancer comes back.

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