There are many signs that you may get before your period is due but all of them can have other factors. Psychologically, if you are trying to get pregnant then you may even think you are experiencing them when in fact you aren't. Early symptoms can include
odd taste in mouth - metallic is usually what is described
Mood swings and irritability
Sore, heavy breasts
Blue veins on breast or chest
Nausea or vomiting
Tiredness
Cramping in stomach
Very light bleeding also known as implantation bleeding.
Please note all of these symptoms can be symptoms of an impending period or an alternative reason that should be checked by a doctor if you aren't pregnant and it persists. Even if you have all of the symptoms, the only way to know for certain is to firstly wait for your period and then to perform an pregnancy test. If you can wait to the date your period is due and then test using an early pregnancy test, you are pretty likely to get an accurate result.