How can I define frost based on a weather record?
The dew point is the temperature at which water condenses from the atmosphere. When the dew point temperature is below the freezing point of water ($0$ $^oC$, $32$ $^oF$) frost forms.

Most weather data includes dew point temperatures. Search your data for when the dew point is below the freezing point of water.

If your data does not you will need to calculate it to find when it is below the freezing point of water. The formulas here will help.