There are several signs that show your period is about to start. One of the signs is that you may experience mood changes often for no apparent reason. You may experience nausea, vomiting, headaches or other flu symptoms. Visit http://www.wikihow.com/Know-when-You'll-Get-Your-First-Period for more details.
For me as a teen, I never got any of the "warning" signs. I actually got cramps the 2nd day of my period which sucked.
Now, as an adult my cycle is more regular so while I still don't get the signs before my period, I know the approximate time of the month (like right now my cycle has the period at the beginning of the month and in a few months it will be at the end of the month). I now get cramps the 1st day, but still not until after I start.
Hey some signs you may be having your period are!: You may start growing pubic hair, and it may start to curl (when your close to period). You may start growing armpit hair. Your breasts may be tender and have a little bump near the nipple. You may get back pains, stomach cramps, and pubic cramps. You will hit a noticable growth spurt an gain a lot of weight. Your forehead will expand. Your body may have more of an odor and your skin may be exceptionally oily. You will start to get more pimples. Your vaginal opening will be larger. You will be moody and feel nauseous with headaches. You may see discharge (white cottage cheese looking moldy milk looking) on your vagina this happens about one year before your period occurs. GOOD LUCK!;) Ps I read a whole big puberty book recently if your wondering how I know all of this.
If you have any color red or dark brown in your underwear you're having your period. I don't get the "warning signs" but some are definitely cramps, headaches, and mood swings. I tend to feel kind of fat in my lower stomach when I have my period. You may also feel irritated easily. Hope I helped! ;))