Online receipt will have "NON-REFUNDABLE" text in caps as below for non-refundable fares.
Also, for domestic fares, the 4th character of fare basis code (found in email receipt) will be a 'Z' , 'P', or 'R' for refundable fares and 'O', 'Q', or 'N' for non-refundable fares.
The 8 letter fare basis codes are found after fare prices in Fare Details section. Below itin has UFUOA0MQ for outbound fare and LAVOA0MQ on return.
The fare rules, which can found online after selecting an itin on delta.com or can be looked on ExpertFlyer from fare basis code will say "CANCELLATIONS PERMITTED." in Penalties section for refundable fares and non-refundable fares will say "
TICKET IS NON-REFUNDABLE IN CASE OF CANCEL."