I don't have a candy thermometer. Is there a visual cue or some other way to know if the brittle is ready for the baking soda?

Before candy thermometers were widely available, our mothers and grandmothers used a different system to measure temperature -- and it still works! :)

The idea is that you drop a small amount of the boiling candy into a glass of cold water and gauge how it reacts. Here's a guide:

http://offthemeathook.com/2011/11/top-tip-tuesday-getting-accurate-candy-temperatures-without-a-thermometer/