If you pull up the trip under "My Trips" at delta.com, it will say whether or not it is refundable. Also, after flight selection and prior to purchase, there is a link that says "Fare Rules" and provides fare rules and details about if the fare is refundable. As noted above, outside of special events, Y fares are almost always refundable. Also, the email receipt will show "NONREF/PENALTY APPLIES" below the price on non-refundable fares (see below).