The easiest way would be to run a pair of wires back from the power input to a 12 volt transducer of your choice (transducer should be available at any electronics shop.) If you're clever, you need only run one wire, and pick up the ground at the switch.
This won't work with self-contained pumps with built-in switches, as the float switch is internal (actually, you could open the thing up and still pick up the input, but it takes a bit more to get there.)
Note that the switches with the telltale light will not be 100% accurate, as they only indicate when power is supposed to be getting to the pump (i.e. they will light when the switch sees the 12V drop, even if there is a wiring fault.) Running wires back will tell you that power is present where it's supposed to be.