Your Basal Body Temperature (BBT) is the best sign to confirm ovulation and pinpoint your ovulation date with a high degree of precision. This is because your BBT responds to the hormone progesterone, which is only present in high amounts after ovulation.
How To Do It
Take your temperature as soon as you wake up in the morning, before doing anything else (including sitting up)
Take your temperature at the same time every morning (or as close to the same time as possible)
Make sure that you’ve had a solid few hours of sleep (at least 3)
This includes getting up in the night to use the washroom
Use a thermometer that records to your temperature with at least one decimal place (preferably 2, there are specific BBT thermometers you can purchase)
Enter your data on a chart or directly into an app (my favourites are Flo and Ovia)
Ovulation = the last dip in temperature before temperature consistently rises
Don't worry too much about the numbers, what's more important is the overall pattern of the chart. Your temperatures should be relatively lower in the follicular phase (between your period and ovulation) and higher in the luteal phase (between ovulation and your next period). Temperatures should on average rise about 0.4 °F or 0.2 °C in the luteal phase, but will fluctuate daily (as you can see on the chart).
After consistently measuring your BBT over a few months, ovulation can be identified as the day before your temperature rises. This allows you to map out your fertile window (or when you are most likely to get pregnant), which can be used to guide achieving a pregnancy or avoiding one! The important thing to remember is that BBT identifies when you ovulated in hindsight (your temperature will rise AFTER ovulation), so if you're using it for the first time or on its own you may miss your fertile window. BBT is can also be combined with other ovulatory identifiers like changes in cervical mucous and/or LH strips. Think of these as tools to help you further understand your cycle and ultimately get back in tune with your body!