With runners on base, an easy way for a pitcher to check for a sacrifice bunt is to attempt a pickoff. A batter will usually turn and show a bunt at the pitcher's first movement. With a runner at second base, it becomes even more likely that the batter will tip his hand if you execute an inside pickoff move.
Remember, you do not have to throw to second base, so that makes this situation that much more valuable.
Finally, realize that if you are able to get the batter to show you he planned on attempting to bunt, doesn't mean that the coach will still have the play on.
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