With adjuvant chemotherapy, that is the usage after surgery for a
non-metastatic cancer, there is really no way of measuring its success,
because it is being used against an undiagnosed threat:  the risk of
recurrence.  So the only real test is to wait and see.

That said, it is common to do a bone scan and CAT scan or similar to
confirm the absence of metastases and also to use as a baseline for
future scans should problems arise.

Tumour markers are detectable substances in the bloodstream which are
given off by tumours.  They are generally also given off by other
processes, and small but variable quantities are normally present.
Their main use is in control of metastatic disease, where they are
present in large quantities, and the rise or fall in the level can be
used as an indicator to the usefulness of the therapy currently being
used.

They are used by some doctors as an indication of the onset of
metastases, but this is generally unreliable as the natural levels are
very variable and the metastases often become symptomatic before the
marker levels show a significant rise.  However the information can be
useful when combined with other approaches.  Obviously if a patient who
has no other definite symptoms shows a large marker rise there would be
good grounds for suspecting metastases were present.

The tumour marker area is one of continuing research, they are
particularly useful for diagnosing hidden cancers such as prostate, and
improved tests are appearing annually, so I have no doubt they will
become more prevalent in this area.

I really ought to put this answer in the FAQ at cancersupporters.com, it
is quite a common question.

Tim Jackson