BMAP* skin;
var UV[3], pixel, AlphaValue;

...
you=NULL;
dist = c_move(me, reldist, NULL, IGNORE_ME|IGNORE_PASSABLE|IGNORE_SPRITES);
if(you!=NULL)   
{  //hit some entity
   c_trace(me.x, you.x, IGNORE_ME|IGNORE_PASSABLE|IGNORE_SPRITES);
   vec_for_uv (UV, you, hitvertex);             //get UV co-ordinate of hit vertex
   skin = bmap_for_entity (you, you.frame);     //get skin at you's current frame
   format = bmap_lock(skin,0);                       //open BMAP for reading and get its format
   pixel = pixel_for_bmap(skin, UV[0], UV[1]);       //get the pixel of the UV co-ordinate
   bmap_unlock(skin);                                //close BMAP now its been read
   pixel_to_vec(NULL, AlphaValue, format, pixel);    //get pixels AlphaValue
   //do something with AlphaValue. Like
   switch(AlphaValue)
   {
      case 10:
         HitArms(you);
         break;
      case 25:
         HitLegs(you);
         break;
      case 50:
         HitTorso(you);
         break;
      case 100:
         HeadShot(you);
         break;
      default:
         Clipped(you);
   }
}
...