How is the direction of Magnetic/Electric Lines of Force Known? It is shown that the direction of magnetic line is from north to the south and that of the electric line is from positive to negative. How do we/scientists know that the imaginary lines of force or the field goes from this to that, what is the meaning of this direction?   
Do not give me mathematical definitions which I already know; I want the fundamental meaning of this.
 A: It's totally a convention.
The direction of the electric field is the direction that a positive charge would accelerate if it were placed there. A negative charge would accelerate in the opposite direction. These things were determined by producing electrically charged objects and measuring which way they move. Like charges repel each other, and opposites attract, so the electric field must point from positive to negative charges, since that's the way that a positive charge wants to move.
But our labels for "positive" and "negative" charges are completely arbitrary. The observation that there are two kinds of charge was made in the 1700s; the labels of "positive" for the charge on hair and "negative" for the charge on rubber when they meet is totally arbitrary. If we had swapped them there would be just one big difference in the world: there wouldn't be people telling electricians that "actually, the current from the battery goes from the negative terminal to the positive" so that they fail their first circuits test when they take physics from a physicist. Electrons in a DC circuit do travel the wires from the negative battery terminal to the positive; but the current, defined as the flow of positive charge, goes opposite the electron flow. Either convention is fine; it's mixing them that's the trouble.
There's another layer of complication when you think about magnetic fields: the cross product and the right-hand rule. Faraday discovered that the magnetic field around a current-carrying wire makes circles. We have to assign a direction to that field. The convention is that if you grab the wire with your right hand, so that your thumb goes in the direction of the current, the magnetic field points in the direction of your fingers. When you go to compute a magnetic force, you use the right-hand rule again. If you came from a country where nobody had a right hand, and you used your left hand for all the cross products, you'd find exactly the same magnitudes and directions for all the physical observables in electromagnetism, like forces, accelerations, and energies. Parallel currents attract each other magnetically; antiparallel currents repel each other.
The mathematical transformation where you change the signs of all the charges in the universe is called "charge conjugation," or $C$. The mathematical transformation where you switch from a right-hand rule to a left-hand rule is called "parity," or $P$. Electromagnetism is completely unchanged under $C$ and $P$ — the directions of the electromagnetic fields are your choice, as long as you're consistent. However the universe is not unchanged under C or P: we are made of positively-charged protons, not negatively-charged antiprotons, and particles in weak decays are born with left-handed spins, and not with right-handed spins. Until you start to ask about the weak interaction, the directions of the fields don't matter at all.
A: The direction is arbitrary. We draw electric field lines with the direction from positive to negative, but it could equally be from negative to positive.
A field line is the trajectory that a test charge would follow if allowed to move freely. So a test positive charge would follow an electric field line from positive to negative while a test negative charge would follow the field line from negative to positive. Making the direction positive to negative is just a convention.
Likewise for magnetic field lines, though this is hypothetical since there are no magnetic monopoles.
