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?

yes. actually the way the caramel behaves when it comes in contact with water will tell you when it is done for brittle. for brittle the caramel needs to be around 300F degrees or hard crack. To know when it is ready, drop a little caramel into cold water and it should separate into little hard balls when in contact with cold water. be careful not to burn the caramel because it will acquire a bitter taste so great in flan but not so much in brittle!