Check the OpenType Features from the Character panel flyout menu. If fractions are listed, and not in parentheses, you can select the text that should be a fraction and apply the feature. This is not something that gets turned on and left on -- it's only used for selected numbers.
Thank you! I have not done much with fractions in InDesign -- I did back in a previous program and use SeriFractions, but it is better to create them the right way!