It is cheating so you have to think very hard about how you handle this one. I personally wouldnt be able to live with myself on that one and would have to tell but I guess most people would feel that if it is a one off mistake maybe you should let it go and if its not a one off you need to think long and hard about what is so wrong in your marriage that you end up behaving in such a way
Depends on the kiss and how the spouse feels. A kiss on the cheek isn't that big of a deal imo.
It all depends how long ago it happened. If its old news, let it be old news. If the secret is eating you alive, then confess and don't do it anymore. Sometimes it is just a kiss, who cares, it doesn't mean anything anyway but the fact is if someone is married, they shouldn't kiss another.
Don't tell your spouse is my advice if you want to stay married. Learn from it and don't do it again.