A flight can only be overbooked until the gate deadline. At that point it may be oversold if there are more properly documented passengers than there are seats.

DL strives to overbook by enough to assure that at the gate every seat is taken. Usually, it is pretty good at it.

Torturing yourself trying to figure out if your flight is overbooked is not generally a productive exercise unless you have a mission critical meeting, you have no status and are booked into an E fare or somesuch. In that case, you might worry a bit.