A good mixing engineer dials things in on their studio monitors, then shops the mix around [playing it on earbuds, car stereo, home theater, bluetooth speakers, etc] before calling it 'done'. How it sounds to one person in their home recording studio is useless if it sounds off or poor everywhere else.
As far as being able to hear for others, I think I generally know a good mix when I hear one. The mix has to be pretty bad for the average Joe/Jane Q Public to notice [I always think back to Metallica's Death Magnetic, and how online fights broke out with people swearing it sounded 'awesome']. So I focus, as described above, on how the playback medium affects the experience versus asking for or assuming 7 billion unwashed opinions.