Uncertainty. Look for the parts of the code where you feel you least sure you have the logic right.
A simple example of this is the bowling game kata: given the throws in a bowling game, calculate the (final) score. The 'hard' part is to keep strikes and spares in mind, including the bonus throws when a strike or spare is scored in the 10th frame.
If you were making an application that would help you keep track of your bowling score, that score calculator would have the highest ROI in terms of testing.
A simple example of this is the bowling game kata: given the throws in a bowling game, calculate the (final) score. The 'hard' part is to keep strikes and spares in mind, including the bonus throws when a strike or spare is scored in the 10th frame.
If you were making an application that would help you keep track of your bowling score, that score calculator would have the highest ROI in terms of testing.