You need to polish the glass with something. It can be done by hand or machine. Klasse AIO by hand with an agressive pad (I've used a Polishin' Pal with an orange pad) seems to work pretty well; if you have the right lighting (like in your garage, looking at the bulb reflection) once you wet the surface with the polish you will see all those spots that you see in the rain, and therefore will be able to see when they are gone, too.
There are many solutions to this, an AIO such as KAIO, ZAIO, Prime, Poli-Seal, or you can just use the same polish you use on your paint, or a dedicated glass polish, or what is that stuff, Barkeeper's Friend or BonAmi or something.
