1) Thinking or saying, "We're just friends."
2) Thinking and daydreaming about the person more and more often
3) Looking forward to the next time you can see and/or talk to the person
4) Wanting to tell them first when something happens in your day
5) Sharing intimate emotions
6) Sharing intimate problems
7) You believe that this person understands you much more than your spouse
8) Keeping secrets and covering up
9) Giving gifts you would not normally give to a friend
10) Spending more and more time alone
I've heard so many people tell me that this was the one that pushed them over the edge.
clipped from: www.beliefnet.com