Instead of OPKs. It's basically a machine that takes daily OPK readings around the fertile time of your cycle can be up to 10 days of readings. Its a tool that uses reliable OPKs memorizes the pattern and in my experience is a good predictor of the right time.
If you consistently used the same brand OPKs on a daily basis and were very careful to compare them you cold achieve a similar result.
If cost is an issue factor in the initial outlay and you will need a box of tests'ish every other month. I think it's as close a tool as you can find to reliably detect OV but it isn't cheap and ultimately having lots of sex regardless of the predictors is the best way forwards.