I guess the algorithm would be something like dew point < temperature but I'm not sure.

Dew point is always lower or equal to the air temperature.

Temperature forecast is near 2 meters height. Your crops / plants are probably more likely to frost when the temperature at the ground surface (10 cm) is close to 0°C (it depends on the plants). This happens even if the temperature at 2m is slightly positive.
Frost occurs especially when the 2-meter temperature is low, the sky is clear at night and there is no wind.
In addition the risk can be increased when the discrepancy between Temperature and Dew point is large and if your fields are at a low level. The cold air flows to the lower areas.
https://www.fao.org/4/y7223e/y7223e07.htm#TopOfPage

Note you can set an alert at a favorite location for low temperature and clear sky and low wind. But it is just a rough estimate because the model resolution is 9km.