How can you be absolutely certain you have never had osteomyelitis?

It depends. You can not be absolutely certain that you never had osteomyelitis because it's possible to have had an infection in the bone (osteomyelitis) or elsewhere that has been effectively treated so that it is no longer there. Frequently, however, the stigmata of the infection remains which shows up as an abnormal on x-rays or with a hot image on bone scan or mri.
Nothing is never. If occurred during early childhood and not informed of infection and if completely healed it is possible not to have knowledge of infection.
You can't. Most of the time you would know if you had a bone infection due to the symptoms that it creates.
Historical review. A review of your history, lab tests and radiology studies can assist in making the determination that osteomyelitis was not present (in the past tense). Negative bone biopsy with negative bone scan (3 phase ) and white cell tagged stuies(indium scan) also speak against it. If that information not available normal present studies/xrays rule aginst it but nevr absolutely certain.