How accurate are ovulation tests usually?

It varies: Ovulation prediction kits are one predictor of ovulation. Others are menstrual cycle length, pain at ovulation, vaginal and cervical fluids. Ultrasound is a great way of detecting ovulation. If everything looks good, ovulation has probably occurred, but the only real test is whether you get pregnant.

OPK: They quite reliably detect urine lh, but much less good for predicting ovulation. Since lh is nature's trigger for ovulation once a follicle is mature, we assume if lh is seen ovulation follows in the next few days. This is true in textbook cycles, but is wildly variable in real life. Ironically, they work best for women with clockwork cycles, and much less well for women who are less predictable.