hall effect switch question

It will work for N or S magnets (I think that's what you mean). If you're asking about output, it's NPN open collector -- current sink only when on, no current source.

If you can measure magnetic field strength in gauss, you can look at the data sheet which is accessible from the link you provided. Otherwise, just get one and measure it with your magnet. By looking at the data sheet, you'll see there's some difference between max. and min. turn-on and turn-off values. If you're looking for extreme positional accuracy, this might not be the part for you. But for most small magnets, the turn-on points for these integrated hall effect sensor/amplifiers should be within a few mm. of each other.

Good luck Chris

So if it's an NPN open collector, it will be turned on only when a magnet is held over the sensor?

Yes. If you use a pullup resistor, the output will go logic low (saturated NPN transistor voltage, usually 0.4V to 0.15V, depending on how much current is sinking) when the magnet gets close to the sensor.

Buy one and try it. See how it works.

Chris