Many interesting points made in this thread. The reason I can compose in notation using Sibelius is the NotePerformer soundset, which makes it possible to get a very good idea of how things will sound just using standard notation. Without Sibelius and NotePerformer I would be forced to go back to composing at a keyboard. Of course, NotePerformer is far short of the quality of VSL and other top-tier sample libraries, so I also do a VSL midi-performance.
Almost all of the great composers composed at the keyboard. The only two I can immediately think of who could compose entirely "in the head" are Mozart and Handel. Even Bach composed at the keyboard. This is not to say that great composers did not do some things "in the mind's ear." Beethoven would take long walks while singing his ideas. Dvorak also was a walking aficionado walking every morning to the local train station and thinking about his composing for the day.
Before recorded music every family created their own music. Every middle-class family owned a piano and most families had several musicians. As a result, composers could easily bring together a string quartet or various wind and brass instruments to try out ideas before making a final commitment to publication. Beethoven in his many notebooks often made notations regarding various "readings" with musicians that he held in his apartment. Bruckner was notorious for making changes to his orchestrations after hearing the initial performance. When I began composing in the 1970's I had numerous musician friends who were willing to try out new music for me. Today there seems to be fewer gifted family and amateur musicians, so getting a reading is probably impossible unless one is connected to a music school. To compensate we have software.
I do believe that with experience and training, one begins to have a firm conception of what various instrument combinations, harmonic progressions, and articulations will work in performance. So the more feedback we receive (as someone previously mentioned) the better we become at composing in the mind's ear.