Nutsedge will have a triangular stem at the base. Quackgrass has fleshy rhizomes that are very pointed at the end. Nutsedge will have small tuber-like nutlets along the rhizome that look like "mini-potatoes". Both are nasty in that just pulling them, the nutsedge leaves the nutlets in the ground and a new plant will emerge from each nutlet; quackgrass acts the same because any piece of rhizome left in the ground will regenerate a new plant. Tenacity required to conquer these beasts!
hortster

The easiest way to learn the difference is to look at pictures of both. Yellow Nutsedge looks nothing like Quack Grass, even when mowed.

Here is a link that might be useful: images of nutsedges