How to measure CAN_H / CAN_L of a CAN Communication using an oscilloscope?
Since you own an MSO oscilloscope, feel free to use two channels, one for H and one for L (like the blue and the red channel in your image). 

The low-tech alternative would be measuring just one channel and trusting the other to be the way it's supposed to be (maybe not the way to go with an MSOscilloscope), or to hook the ground-clip to the low channel while probing the high channel. You then only see a difference in voltage of 3.5V - 1.5V = 2V when H is high and L is low. 

However, if you have the option, use two separate channels. Maybe your scope even does debugging!