  We investigate the quasiparticle interference in the heavy Fermion
superconductor CeCoIn_5 as direct method to confirm the d-wave gap symmetry.
The ambiguity between d_{xy} and d_{x^2-y^2} symmetry remaining from earlier
specific heat and thermal transport investigations has been resolved in favor
of the latter by the observation of a spin resonance that can occur only in
d_{x^2-y^2} symmetry. However these methods are all indirect and depend
considerably on theoretical interpretation. Here we propose that quasiparticle
interference (QPI) spectroscopy by scanning tunneling microscopy (STM) can give
a direct fingerprint of the superconducting gap in real space which may lead to
a definite conclusion on its symmetry for CeCoIn_5 and related 115 compounds.
The QPI pattern for both magnetic and nonmagnetic impurities is calculated for
the possible d-wave symmetries and characteristic differences are found that
may be identified by STM method.
