Going by experience, you will definitely know when you find a girl you want to be exclusive with. My current girlfriend, I knew after the 1st date. I didn't go exclusive right away but within a month I had no interest in seeing any other women.
You should be honest with her. A lot of women have gotten emotionally attached to me. They deserve you to be honest about your intentions and I always am.
There was one that was attractive and a great lay. She knew the deal and when I met the aforementioned current girlfriend, did not take it well at all when I cut her off! I was clear on the deal, she still wanted in my pants, and was obviously still hurt. Sometimes you just can't win - plates will get hurt but I have a clean conscience.