I have this handy weed-recognition tip. When planting a new thing from seed, only plant it in one area. Then, when faced with a puzzling sprout in that area, you need only check whether the same type of sprout exists in other places in your yard. If so, it's a weed. A variation on this is to plant things in lines or other geometric shapes, but that looks kind of dumb for flowers, so I don't do it much. After the first year, you'll probably recognize most of the plants.