There are a number of commonly used ways to predict when you’re ovulating and maximise your chances of conceiving a girl. These include:

Ovulation calendar -
You can use an ovulation calendar to predict ovulation if you have regular periods.

Basal Thermometer -
You can use a basal body thermometer to detect the small temperate increase which happens when a woman releases an egg. The egg release stimulates the production of progesterone, which raises body temperature. This method doesn’t actually predict ovulation, but shows that you’ve actually ovulated.

Ovulation kits or ovulation tests -
Ovulation kits work by detecting an LH-surge in the body, which happens just before ovulation occurs. These tests are without doubt the easiest and most accurate way to predict ovulation, and are cheaply available in chemists / pharmacies or online.

For the highest degree of accuracy, therefore, How to Conceive a Girl recommends that you use an ovulation test to predict ovulation.