I had a smiley face saying im positive on opk how do i know this is the first day not the last day?

i am on my first round of clomid and today 5-7-11 i had a smiley face on my digital ovulation test, but how do i know if this is my first day of lh surge showing up or my last day. As long as i got a smiley face does that mean i need to have sex as soon as i can, not knowing exactly if this is my first day. And what is my chances if conceiving??