In short, not easily.  Mostly it is a matter of wait and see.  There are
things that can be done to detect metastases earlier, but unlike primary
cancer there is little benefit from early detection, and so there is a
school of thought that it is best to leave well alone until symptoms
appear, if they do.  If we ran the tests (most of which involve harmful
radiation doses to the body) on everyone who had an early stage cancer
removed we would end up doing more harm than good, as well as costing a
lot more.

Tests that can be done is a CAT scan or similar to search for unexpected
growths in the body, an isotope bone scan to look for inflammation sites
in the skeleton, and blood tests to look for "tumour markers", compounds
characteristically released by tumours.  Some institutions are prone to
carrying out all these tests on a "just in case" basis, and to protect
themselves against possible legal action, even when there is not really
a clinical basis for ordering them.  It would be normal to do some or
all of these if the primary tumour was at high risk of having
metastasised, eg stage III.

Tumour marker tests in particular, while fairly cheap, and minimally
invasive, are rarely useful in making an initial diagnosis of secondary
cancer because the background levels of the marker compounds are very
variable.  What they are good at is monitoring the effectiveness of
treatments for a known tumour, where the level is already very high and
one watches the rate of change, rather than the absolute level, of the
marker.  But new tests are being developed which are better for initial
diagnosis, and the above may be less true in future.

Tim Jackson

Thank you for the information!