I have two such friends, my two unofficial specialists on the two demographics I'm aiming for in just about everything I write. I'm super blessed in that department.
My first is my story's spiritual consultant, who pinpoints just how out of line or how spot-on I am in what I'm saying when it comes to apologetics and Christian sensibility. He and I plan to go to seminary together, but having been his friend most of my life, I can testify that he knows his stuff. He usually tells me, "This might lead someone astray in their walk, you might want to rethink this, try this angle," or "You're on the right track with this theme, but maybe if you push this aspect a little harder and lay off this other one, you're point will be clearer..." etc., etc. He's always one to streamline my works so that their messages reach easier and with a lot more impact...so I've been told, anyway. He's also a closet nerd, so he occasionally has an idea on what would be "cool," emotionally-fulfilling for a reader.
The next, who is almost an antithesis of the first, is my unofficial specialist in how fellow "nerds" will recieve my works. He knows his inner-geek, let me tell you: he channels that fantasy-sci-fi-speculative-fiction-genre-loving-demographic like he were every fan of that genre conglomerized into one man. He also has his insights on romance, character dynamic, and the like, so he's pretty indispensable when it comes to story development. Most prominent, maybe, is his ability to just inherantly "know" what action is needed to make a story more excited. Not necessarilly fighting or battles, but conflict and suspense and in general. He can be a little overzealous, though. I usually go to him first, so that once I go to my other friend, I can tweak his overactive input and make it Biblically sound.
Usually, that amounts to a pretty good balance. USUALLY.