Every little info will help.
Most Helpful Girl
How to know someone is on their period?
Well the most obvious thing is the mood - she is likely to snap or get angry over the tiniest thing - [happens to me]. Also, notice for signs of grimaces - as a few days before she starts her menstrual cycle - she is likely to feel jolts of pain - often they are the clue to women they are starting their monthly cycle. She'll feel crap - won't want to get out of bed.
She will also wear dark colors in case she is worried she may get blood on her white pants,etc
She would also make trips to the bathroom more often.
She would also walk funny due to period cramps (someone get them).
What you should do?
Help her with tasks - so she feels relaxed
Don't sympathetic - don't say something that can make her feel annoyed - be patient
Ask her how she would like to be treated - since; no-one knows her better than herself
How to treat her?
Get her chocolate - believe me whether you get her chocolate ice-cream or a bar of chocolate it has a calming affect on women in their monthly cycle for some unknown reason.
Give her painkillers - if she hasn't taken any.
Watch her favorite movie, with a bowl of chocolate ice-cream, lights out - (I normally do this and it helps - takes the mind of the pain).
Give her a massage - they can be soothing.