That's one thing I'm still learning at this point. Best I can figure out... some ladies will stick their fingers in near their cervix (but I haven't figured out where mine is)... So I just go by what I see in my underwear when I see any.
If what you find seems sticky, or findings are scant, you're probably not ovulating yet.
If what you find is creamy, ovulation may be coming, but not just yet.
If what you find is wet, watery, and slightly stretchy, ovulation is very likely close. Find time for some baby-making sex.
If what you find is very wet, stretches between your fingers for an inch or more, and resembles raw egg white, your cervical mucus is very fertile. Ovulation is right around the corner, and now is the ideal time for intercourse.