Voltage level

If a lamp is connected to an electric circuit with a switch, it lights up if the switch is turned on. This is interpreted as the logical state "1".
The CAN bus works exactly the opposite (negative logic). If no device sends signals to the bus it is the state "1". If a "0" is sent it surpresses the "1".

They say for this:

The logical "0" is dominant; the logical "1" is recessive.

 

In the following test a two-channel oscilloscope is used. An oscillogram of a typical CAN bus is shown.
The voltage level with which the CAN bus displays both logical states can be determined based on this oscillogram.