Bil Simser posted on a problem with CAML where he mentioned reverse engineering the internal field name to the original display name that created it.  I had to do just this thing for one of migration utilities.  Here's the code (C#):

  // Fixup an internal name back to the regular way that it should be so that we can get the same
  // internal name back when we create a title.
  private static string FixInternalName(string internalName)
  {
   // Get the most common one out of the way as quickly as possible
   string f1 = internalName.Replace("_x0020_", " "); 
   int pos;
   // while we have more processing to do
   while ((pos = f1.IndexOf("_")) >= 0)
   {
    int chrVal;
    if (pos + 6 > f1.Length)
    {
     chrVal = PartialEncoding(f1,pos);
    }
    else
    {
     chrVal = int.Parse(f1.Substring(pos+2, 4), System.Globalization.NumberStyles.HexNumber);
    }

     if (f1.Length > pos + 7)
    {
     // char isn't at the end of the string
     f1 = f1.Substring(0, pos) + Convert.ToChar(chrVal) + f1.Substring(pos+7);
    }
    else
    {
     f1 = f1.Substring(0, pos) + Convert.ToChar(chrVal);
    }
   }
   return (f1);
  }

  private static char PartialEncoding(string fix, int pos)
  {
   pos += "_x".Length;
   if (fix.Length > pos)
   {
    string nw = String.Empty;
    while(pos < fix.Length)
    {
     nw = nw + fix[pos];
     pos++;
    }
    while(nw.Length < 4)
    {
     nw = nw + '0';
    }
    return (Convert.ToChar(int.Parse(nw, System.Globalization.NumberStyles.HexNumber)));
   }
   else
   {
    // Not even enough to start -- return a space it will get encoded
    return ' ';
   }
  }