I get bloated 3-6 days before like a fat whale and my appetite increases, then I have a REALLY skinny day then it comes. Happy days. The grumbling pain before hand and increased back pain also indicate it's coming...
Stomach Aches about 3 days before. They Kill especially in the middle of exams lol @ me clutching my stomach in the middle of my maths exam and people just looking at me like what the hell is she doing lol.
I only know I'm going to come on when I've got a huge craving for a big chocolate bar before bed. The next morning I know I will come on when I go to the toilet. And every 8 months I get a stomach ache that morning. Simple as that really.
I get a weird period-type pain a couple of days before usually. Kind of reminds me I'm due on soon. Sometimes notice my tummy is a bit rounder than normal. Also, if I catch myself in tears over something stupid (I rarely cry normally) then I know my period will come the next day!
i'm on the pill so that already takes me to the day, and depending on how much time difference there was on taking it every day then i already know to the hour, but if you're talking actual signs then about 10 minutes before i get a slight cramp.
My breasts hurt and I might get abdominal pain a week before it's started (rarely though). The biggest sign is, my boobs look bigger than usual (and I think they've grown when my period is just going to start) and they hurt like anything. They feel like...rocks in a bean bag and if anything just touches em, even lightly, they hurt for about 20 minutes.
And that's always the time people want to hug me really really tightly and don't get why I don't want a hug and get annoyed.