How can you know if an infection from an abscess tooth has spread to the blood stream?

Fever? If you feel feverish and malaise it is likely yhe infection has spread.
Treat abscess. An abscess or infection most likely will invade the surrounding blood vessels and enter your blood stream so please see your dentist to eliminate the abscess. Antibiotics are used for that very reason it's not only for the abscess but to control the infection which has gotten into your bloodstream.