In smoothieware you can use the beta value to determine temperature from thermistor readings. The 25/50C beta value for the Alirubber heat pads is 3950. This means that using the beta method, your temperatures will be accurate up to about 60 deg C or so. Above that, the temperature readings will be approximately 5 deg higher. As this is for the heated bed, this error is generally acceptable.
I just tweaked the beta number using a fluke with a K-type thermocouple to be accurate for the top of the aluminum heat spreader taped down (with thermal compound) at 70C to 120C. But they have cheap multimeters out there with thermocouples for like $20 that should get you really close.