Use iccgamut to create a .gam file and a 3D HTML document of an RGB working space profile:
iccgamut -v -w -ff -ir rgb_profile.icc
View the gamut:
http://www.idea-digital.com/examples/AdobeRGB1998.x3d.html
Repeat again using a destination CMYK profile:
iccgamut -v -w -ff -ir cmyk_profile.icc
View the gamut:
http://www.idea-digital.com/examples/ISOcoated_v2_300_eci.x3d.html
Then use viewgam to compare the two gamut files with the larger RGB gamut shown as a wireframe:
viewgam cmyk_profile.gam -w rgb_profile.gam output_comparison.gam
View the gamuts:
http://www.idea-digital.com/examples/aRGB_v_FOGRA39.gam.x3d.html