If you have information in Boxes 1b and 1c, you will be able to tell which box to mark.

If the date in Box 1b is a more than a year before the date in Box 1c, then the gain or loss is long-term. 

If the dates are a year or less apart, the gain or loss is short-term.

Click here for more information about Short-term vs Long-term Capital Gains.