There's not really an easy way to find these, but there are some rules of thumb you can keep in mind. For one thing, they will be easier to find during the winter, since the widebody equipment (where most D1 seats are found) is in high demand to fly internationally, especially to Europe, in the summer. In January I see (via a subscription service I have access to) a random smattering of widebodies on routes like ATL-DTW, ATL-JFK, ATL-MCO, ATL-MSP, ATL-PDX, ATL-SEA, ATL-SLC, and BOS-MCO.
Also during off-peak times, there are sometimes widebody service to/from major events, like to and from the Super Bowl host city or major conventions in Las Vegas.
But, you have to manually look at the aircraft type for each flight, because one-off flights like these are generally not marketed as Delta One (even though they have the seats), so you wouldn't notice them just by looking at the price grid on Delta.com. If you have a service like ExpertFlyer you can get an easier-to-digest-at-a-glance view of all flights and their aircraft types for a day which might help, but again these arise relatively infrequently so I wouldn't get EF just for that.