So the setup is like this: you have a plate on one side that is the emitter plate - you shine light on it and if the energy of the photon is greater than the work-function of the metal, an electron will be emitted and will travel across the gap between the plates to the collector plate that is on the other side. Any photon with energy greater than the work function will eject an electron. So simply knowing that an electron is emitted won't determine the workfunction or the kinetic energy of the electrons. However, since electrons are charged particles, if you increase the potential difference (voltage) on the collector plate an electric field that opposes the flow of electrons from the emitter to the collector is created.
Electrons ejected from the emitter plate have kinetic energy that is the difference between the workfunction and the photon energy. If there is no electric field between the emitter plate and the collector plate, after emission the electron travels at a constant velocity across the gap. However, if there is an electric field, then the electron will experience a force that, assuming the collector plate has a negative charge, will cause the electron to slow. If the electric field is strong enough, it will bring the electron to a stop and reverse it's direction back to the emitter plate. The force the electron feels is F = eE where e is the electron charge and E is the electric field.
You determine the maximum kinetic energy by increasing the voltage (which creates the electric field) to the point where the electrons are brought to a stop just before hitting the collector plate. You know the force (F=eE), so you know the acceleration (F = ma = eE), you know the distance between the plates (part of the experimental setup), you know the final velocity (=0), so the initial velocity and kinetic energy can be calculated.