How do i know if my tooth infection could spread to my brain?

You already know: That a tooth infection could spread to your brain. If you are asking has the infection spread to your brain, the answer is probably no, especially if you are on the internet asking questions and not in a coma. Get that infected tooth treated promptly.

You don't: You don't know. That's why you need to get the infection treated now. By the time you figure out that the infection has gone that far it's too late. Knowing that the infection could spread to your brain, the question is why would you wait?