The email receipt would confirm what you booked in the Fare Details: section which has the fare components breakdown. Almost certainly it's a combination of DFW-LAX and LAX-DFW fares as you suspect which is not a broken fare (it's the opposite). A broken fare would be if you had separate fares for DFW-SLC and SLC-LAX segments (which would cost considerably more than the single DFW-LAX fare you actually booked). There isn't really such a thing as a single "roundtrip" fare. There are still two fares involved, it's just that there can be cheaper fares that require a roundtrip booking (although this route likely doesn't have any since DL competes with WN on it and WN doesn't have them -- DFW-LAX fares are just cheap because of all the competition on the route). This thread suggests they won't short check unless connection is over 6 hours -- Short check bags. Perhaps they might let you do it if you explain you are not doing this for hidden-city booking purposes, but don't really know how much discretion the agents have.