well just in general you normally should place a marker at the beginning of the row when you join after casting on. usually placing one of a different color than the others you will be using is helpful so you know which one is the beginning of the row.
on those occasions when i have been too lazy to deal with cutting a piece of yarn or to look for a marker, i have just followed the line up from where my cast on tail is and gone with that. it may be off a stitch or two but i am usually content with it being "close enough."
now if that is different in your situation, i don't know.