One test you can do on the rear sprocket is to pull on the chain at the very rear. If the chain and sprocket are a good match you will only be able to lift the chain a little bit away from the sprocket. If the sprocket is worn you'll be able to lift it a lot and may well see daylight between the chain and sprocket.
OK so i did this, and i can see the chain move on the sprocket. but only the half of the link that im pulling on moves away from the sproket. To form a little triangle with the 1/2 link its attached to.
The other half of that links stays in contact with the sprocket, is that worn?