I'm not aware of what in the fare code or rules would indicate a fare as an exception or consolidator fare. However, if a fare offered is cheaper than the lowest available fare on Delta.com (especially if the fare class is the same), you can be pretty certain that it is in fact an exception fare.

As others have pointed out, travel agents, unlike airlines, are not required to abide by the DOT's 24-hour risk-free cancellation rule. So, my advice is to confirm that the travel agent you are using voluntarily offers a risk-free cancellation policy and then I would call the airline after booking to confirm if the fare is an exception fare.