*Well you can have a look at your cervical mucus, have you ever noticed at time you get 'gunk' in your pants? Sometimes it is more stringy. You can chart it, look up 'Billings'
You can also chart your temperature, but you have to be accurate, not sure how well electronic thermometers are, you used to use a rectal thermometer.*
You just need a basal body temperature thermometer which has a finer scale than normal thermometers and can record smaller changes in temperature
You can use this orally, but has to be taken first thing in the morning at rest, before you get up. Doesn't need to be vaginally or rectally
Not everyone gets EWCM, or if you have irregular cycles you're very likely to not be able to use cervical mucus as a gauge, as if you have anovulatory cycles you may have erratic patterns of CM. Even temping with anovulatory cycles doesn't help you to predict when your period will be. You can only predict when your period will be if you ovulate and get a temp rise, so can see it fall before your period arrives
If you have cycles like mine then temping doesn't help at all - I don't ovulate, so my temps are like a mountain range, and my period could come at any time between 40 days and 4 months, or longer