Measuring a 2-pin active wheel speed sensor without having it connected to a vehicle
A 2 wire automotive speed sender can only really be 2 types

Type 1 is a inductive sensor. It has a magnetic feild and metal moving through it generates a tiny voltage dependant on the speed. Usually signal levels are not higher than 1V AC when measuted with a multimeter. And should be buffered by an op amp if you wish to use it in your circuit. 

Type 2 is a hall effect sensor. They do not commonly use this type in 2 wire. But they exist. They have a hall effect sensor powered by an external pullup on the signal pin. When metal is detected it pulls the signal pin low. All the way to 0V for a short period with some internal capacitance. Or more commonly to about 2V

How to tell the 2 apart? The inductive type will have a fixed resistance if there is no power to it. The hall effect type normally would use a 10K pullup to 12V and should not be damaged by a reversed voltage provided that pullup resistor is used to limit current.