The only way to know efficiency accurately is to build the entire robot and measure it . . . so since that isn't possible, you have to estimate based on basic principles.
For example, typical gears have 90% efficiency. If your robot has high friction on the ground, thats another loss. Got tracks? Another efficiency loss.
If you have high precision mechanical components, and your robot is well designed, you may get 85% efficiency. But if you hand make everything, expect more like 70% efficiency . . .
Does it depend on the quality of the motor??
Nope, because this is factored in already on the motor output specs. The torque they give on the datasheet should be what comes out the motor.