Seeing if a column value is updated

I have a complex user model with a validation method

before_validation_on_update :geocode_places

This validation geocodes the "places" column in my user table each time an update occurs.

The problem is that this validation takes quite a long time. Is there any way to only call this validation when the places column is updated? Sort of like a "before_validation_on_column_update" so to speak..