Well, for distinguishing between onions and daffodils you can always crush the foliage and smell it. The scent is oniony and a daffodil's is not.

As for ridding your flower beds of them, roundup is a good choice. Injuring the foliage with a weedeater and/or using a little dishwashing liquid or spreader sticker helps. If they are coming up in the middle of things you can use a sponge or cotton gloves on top of rubber ones to apply by hand. If they are only around the woody part of your hydrangeas you can spray them point blank on a windless day. Even if you do get a little overspray or drift, Roundup won't kill woody plants at the weed spray concentration, though you could have disfigured, discolored foliage if it gets on it. Roundup (glyphosate) does not move through the soil so it won't hurt anything that way.

Good luck!!