How can i tell if a tooth infection is serious and is spreading and how can i tell if its spread to the brain or something?

There may be no symptoms. But if there are, they could include continuing or increasing pain, swelling or fever. If an infection had reached your brain, you would be exceedingly ill with any combination of high fever, headache, altered consciousness, etc.

It is very rare that a tooth infection spreads to the brain if treated when it first occurs. It would have to go untreated for weeks or months to even have a chance of spreading to the brain and even then it would be rare. If you have a tooth infection, see your doctor and get it treated.

You can't tell, only a dentist can. An infection is serious so best advice is to see a dentist for exam and x-rays and necessary treatment. An infection will spread if not treated.