How table fan's blades always move in the same direction even when direction of electric current is changed by changing terminals (+ve, -ve) of source?

Well for the most part you cant tell, as these cables are by design just long strings of conducting materials. With the focus on redundancy they are set in a mesh so that even if a cable fails, the power can rerouted via other cables to reach the load.

The same cables can be used for incoming and outgoing power. Thus you cant tell if they are incoming or outgoing lines.

But there is a trick as most of the power transmitted is a higher voltage than the final distribution voltage, you can look for transformer near you and if you see one, then the cable which is smaller in size and at lower height will have outgoing power.

The equipment you can use to measure is a Ammeter but they usually work at low voltages for higher voltages you will need  CT, these are both non feasible methods.

That is the best of my knowledge, i dont know if there are other methods.