Think about what must happen in order for a runner that looks lousy The Stable Guys in the racing form or past performances to win the race. Obviously it is a surprise to many of the horse players. Why? Because it has been running poorly, that's why. It's not hard to tell which horse has been running poorly, simply look at the odds board and you'll see the highest odds on the horses that finished the worst in their last races.
The best handicappers look very closely at those horses and their running lines to see if there are any clues that will signal a reversal. In most cases there are some clues that you can find. First and foremost, that reversal will come because something has changed. A drop in class is a big change that often helps a runner to win, but most horse players are aware of that and you won't usually find high odds on a horse that is dropping unless it looks really bad.