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?

The other rules: Use plenty of ice water for the test, but the candy shouldn't touch any ice. Use a clean, dry spoon for every test. I found a stack iced tea spoons are perfect. Test before you think it's done. Knowing what the soft ball and hard ball stages feel like will help you know the soft and hard crack stages are.

Periodically test the hot sugar mixture by dripping some into a glass of ice water. If the droplet holds together and sinks to the bottom, fish it out and see how solid it is. It does take a while for it to get to the "hard crack" stage--which I am assuming the brittle recipe calls for...but I have never tested for this without a thermometer so I don't know all the subtleties of what it should feel/look like. I bet you could look it up!

Sorry I'm not more help.