Like btbas said you need an osiloscope to do measurements. A simple multimeter won't suffice.
Also remember CAN uses a differential scheme for signalling.
A dominant bit (usually 0) is the very little voltage differential between CAN_H and CAN_L. At that time both CAN_H and CAN_L will be around 2.5 (I don't remember the exact values, check the specs).
For recessive bit (usually 1) differential of CAN_H and CAN_L will be I think 2.2V min (check the specs again for exact values) but CAN_L would nor be 0. It woyld be around 1.2v.
Moreover take care of bus termination while doung the measurements.
This is how i measured the voltage across the bus CANH -CRO channel 1 and Connected the Ground to CANL - By this i got the differential voltage as 2.1V for Dominant Bit and <200millivolt for Recessive Bit.
You can individually measure the CANH Voltage and CANL voltage and subtract the same to get the differential voltage. Voltage >= 2 is recognized as LOGIC 0 and Voltage <500millivolt is recognized as LOGIC 1 in the BUS
Note: This voltage depends on the Standard used i am using J1939Standard.
Note: To measure the voltage Send an Identifier with lowest value i used 0x007.