How can I measure latency in a CAN bus?
With a multichannel logic analyzer (or scope) you can measure the CANH/CANL signals and the signal driving the motor and calculate the difference.

See below for a very cheap one. Logic analyzers (like below) also have the possibility to interpret communication protocols like CAN, SPI, U(S)ART, I2C, MIDI, DMX etc.

This one has eight channels, if you want to measure the CANL/CANH and probably multiple signals per motor driver simultaneously, you could use two analyzers and use CANL/CANH for both analyzers (i.e. connect CANL/CANH to two signal lines from both analyzers).