We experimentally investigate the modal dependence of the guided-acoustic wave Brillouin scattering (GAWBS) characteristics in a few-mode fiber (FMF). We clarify that launching higher-order modes into FMF induces different peak frequencies from those of single-mode fiber (SMF) in the GAWBS spectrum and that the GAWBS spectrum strongly depends on the mode excitation power ratio between the LP(01) and LP(11) modes. We evaluate the mode excitation power ratio at a splice between an FMF and an SMF by utilizing the GAWBS spectrum, which we realize by detecting only the fundamental mode at the output of the FMF and without the use of a mode demultiplexer.