@ElizaBennett - That's a good point that it can be tricky to tell.
But for some women, taking the basal body temperature is not a good option. If you wake up at different times every day for whatever reason, or do shift work, you won't be able to see the temperature shift as clearly.
In that case, checking your cervix can be your second sign. Before ovulation, the cervix will gradually soften and drop, and then after ovulation it will slam closed and get high and hard again (I might be mixing up the high and low).