The usual test for Hepatitis C looks for the antibody.  If she tests positive
for the antibody, the next step may be to look for the virus itself.  This
can be done with a Western Blot test.  If she only tests positive for the
antibody, but not for the virus, there would be no risk of transmission.
 There would be no virus to transmit.

It may be well worth the money to pay for the Western Blot in terms of peace
of mind.  I have yet to hear of a nursing mother who was positive for the
virus.

Elizabeth Puzar, IBCLC