Considering that bacteremia has been demonstrated from flossing,
brushing, and even just eating (in the presence of periodontal disease)
it is hard to imagine how any bacteremia can be attributed to any
specific tooth, absent a surgical procedure.  The latest recommendations
of the AHA have vastly curtailed the indications for antibiotics, and in
the UK there is a recommendation to eliminate antibiotic prophylaxis for
dental procedures altogether.  Apparently any bacteremia is not
considered a significant risk (though I would like to see the rationale
for this change).

Steve

They take a blood sample before and after the dental procedure and
they culture it. It's a complex procedure though. Here's an abstract
to give you an idea.

Prevalence, duration and aetiology of bacteraemia following dental
extractions.
Tomás I, Alvarez M, Limeres J, Potel C, Medina J, Diz P.

Department of Special Needs, School of Medicine and Dentistry,
Santiago de Compostela University, Santiago de Compostela, and
Research Laboratory, Department of Clinical Microbiology, Xeral-Cíes
Hospital, Vigo, Spain.

OBJECTIVE: To investigate the prevalence, duration and aetiology of
bacteraemia following dental extractions, analysing the factors
affecting its development. SUBJECTS AND METHODS: The study group was
composed of 53 patients undergoing dental extractions under general
anaesthesia. Peripheral venous blood samples were collected at
baseline and at 30 s, 15 min and 1 h after the dental extractions.
Samples were inoculated into BACTEC PLUS aerobic and anaerobic blood
culture bottles and were processed in Bactec 9240. Subculture and
further identification of the bacteria isolated was performed by
conventional microbiological techniques. RESULTS: The prevalence of
bacteraemia following dental extractions was 96.2% at 30 s, 64.2% at
15 min and 20% at 1 h after completing the surgical procedure. The
bacteria most frequently identified in the positive blood cultures
were Streptococcus spp. (63.8%), particularly Streptococcus viridans.
CONCLUSIONS: In our series, the majority of patients undergoing dental
extractions developed bacteraemia, usually of a streptococcal nature,
independently of the grade of oral health and of the number of
extractions performed. Positive blood cultures persisted for at least
1 h after the dental procedure in a considerable number of patients,
questioning the supposedly transient nature of bacteraemia following
dental extractions.

Low-level bacteraemia following RCT would be very difficult to prove,
since the bacteraemia may well be caused by other oral factors like
oral hygiene and periodontal disease. There is no practical way of
monitoring bacteraemia, if that's what you're concerned about. Your
body is naturally adapted to cohabiting with a wide variety of
bacteria and you can help maintain strong natural defences by living
healthily.

Regards,
George