COURTESY RUNNER APPEAL:

When a courtesy runner is asked for and the courtesy runner is NOT the last out, anytime AFTER the first pitch to the next batter, the defense may APPEAL that the incorrect courtesy runner has been substituted for the previous batter. If the courtesy runner is NOT the last out, the resulting penalty is that the courtesy runner is OUT. If the courtesy runner IS the last out, there is no penalty to the defense for the appeal. If the batter hits the first pitch, the defense may make the appeal after the completion of the play and BEFORE the first pitch to the next batter. 

Keep track of the opposing team’s last outs! .