Chris Brady's site is a great resource:
Ice & Rain
Note that you can see ice under the windscreen wiper in gin-clear conditions due to water collection under the blade and ambient air temperature. So it's not a useful indication of airframe icing. The centre of the windscreen is a great indication of rapidly accumulating airframe icing and it will be more obvious than the wiper bolt if the onset is rapid and severe (in such cases, the wing leading edge may not even be visibile from the flightdeck due to the thickness of the cloud).