You don't say where you live so I don't how hot it is, but most folks divide their hostas in early spring when the leaves are about 1" or so out of the ground.* This site has some good info under 'Division'.* One thing it doesn't mention is that it's best to have at least 3 offsets in the group you replant.* It's best if you can keep them together in one piece.
Here's some great pictures.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.