The gamut is the range of colours that can be printed (or displayed on screen) by a given device. The profile contains the information (if you are using an ink and paper combination which has a profile, if the manufacturer even bothers supplying profiles for their device) but getting PS to show which colours are out of gamut for a particular RGB profile is tricky. Just previewing should do it, but you have to have confidence in your screen profile to be sure that what you are seeing is the result of clipping (colours that can't be produced being replaced with those that can). You could try printing some gradients and see where they break down and start being muddier than the the on-screen version, as a warning of where the danger zones are.