As others have mentioned, speed loss is the 1st indication of airframe icing. It part of my SOP to dial in the TAS and monitor it during the flight. I know what TAS to expect for various configurations. If my TAS isn't correct (too fast, too slow) then you know something is wrong such as dragging a cowl flap, fuel flow incorrect, or in this case ice building on the airframe.
It doesn't take much ice to cause a noticeable drop in TAS. And yeah, the ice banging on the side of the nose really gets your attention!