Consider a hypothetical bus drifting along at a uniform speed in the vaccum and a hypothetical insect which crawls inside the bus without any physical contact with it. I think in a moving bus the insect suffers lot of crashes, which can be used to know whether the bus is in motion or not. Instead of the insect, robotic model can be used which stands still at a particular point inside the bus, to have better results. I hope the above experiment is neither optical nor electrical. So, according to this thought experiment. I think we can tell, by making mechanical experiments (other than optical or electrical), whether the system is moving or not.