If recipe calls for brown sugar how do you know light or dark

Light is considered the default where none is specified. Sometimes it is interchangeable, but sometimes you want the stronger molasses flavor of dark and sometimes it isn't as pleasant.

Ddvc

In my experience, a recipe calling for "brown sugar" usually means light brown sugar. Dark brown sugar is usually called for specifically.

It generally shouldn't matter. Use whatever you have on hand or would prefer to pick up.