Is there a clear ruling on when menses start?

السلام عليكم!

Is there a ruling on the start of menses? I'm not sure where the line is but would need to make the choice for fasting. (Suhoor just ended in my town.) I need to choose betwen fasting or not and fajr or not, but as I have no heavy flow yet but have seen smller signs indicateing menses starting today / have started, what to I do?