Commonly available ovulation predictors (OPK, digital monitors, saliva microscopes) may be very effective in detecting fertile days, but they're not able to determine whether ovulation has really taken place. They don't confirm the occurrence of ovulation, only that your body is preparing for it.
Most of these predictors detect increases in LH level, which indicates impending ovulation, but not its occurrence. In some cases the body may prepare itself for ovulation (it produces the hormones, which cause the symptoms of fertility to occur, i.e. fertile mucus, the fertile position of the cervix, or a positive ovulation test), but ovulation does not occur (e.g. because of stress or illness).
In these instances, only basal body temperature can confirm that ovulation did not occur (the egg will not get released, progesterone will not start to act and the basal body temperature will not increase). If you want to know whether ovulation really did occur, even if you do an ovulation test, make sure to also take your temperature (Is basal body temperature a reliable way to indicate the occurrence of ovulation?), or try USG monitoring.