As FvM mentions, it can be determined, but you require to determine the kw constant and the Rs. And to be able to measure the actual applied voltage at the motor terminals, as both the H-bridge and the current-sensing resistor will have some voltage loss.
You don't mention how large your motor is. In larger motors, Rs is quite small, and you'll require a 4-wire resistance measurement to determine it accurately.
The kw constant, unless you can get it from the manufacturer, you will require to rotate the motor at a precisely known speed, and measure the open circuit terminal voltage.