There are two basic way to test a system to ensure that the control(s) will work when snow and/or ice are detected.
The first way is to simulate the conditions required for operation. Lower the temperature of the sensor if it is not cold enough out and then get the sensor wet. You can refer to Why isn’t my sensor turning on my system when it is snowing outside? for instruction on testing your system via the sensor.
If you do not have a sensor or if you have reason to suspect that your sensor may be bad you can take a more direct approach. By shorting signal to ground at the sensor input in the control panel you can simulate a call for heat. The system will respond as if the sensor were seeing snow conditions. The snow and heat indicators will come on and relay or contactor will be pulled in. Click on the examples below to see how sensors connect to each control type.