A capacity sensor detects the contents of the container it's next to. (Like a Storage Bridge, you don't have to wire it to the container. It should highlight the container it is sensing, when you're placing it.) The sensor has two red circle outputs. Scanning the sensor shows what these are: 1) Partial Fill, which sends a signal if at least one storage slot has an item in it; and 2) Complete Fill, which sends a signal if all of the slots have something in them--regardless of whether the stacks in the slot are full or not.
Item Utilities (Electronics tree).