How could i tell or suspect tooth infection is spreading. ie to brain, heart, blood stream, down into throat.

See a dentist: See a dentist who can evaluate you, whether or not you have a dental infection and what treatment and precautions are indicated. This is not something that we can evaluate and assess over the internet. You may not have signs in early stages so don't take any chances.

Obvious signs: of systemic toxicity, such as chills and spiking fevers and classic local signs of tooth infection, like swelling and pain.

Spreading : Hard to tell. If you are experiencing worse symptoms, pain swelling then it could be spreading. Make sure you see a doctor or that you are on an antibiotic.