Kevin, the-rajah is correct.
Unfortunately, it's not possible for anyone to accurately quantify the risk of passing or contracting oral herpes for a given situation. In terms of likelihood, oral herpes is likely not contagious when there are no symptoms. There are several days a year that the virus can be present without symptoms (called asymptomatic shedding or asymptomatic reactivation) and a person is contagious during this time. A person wouldn't know when these days occur, but the days are very infrequent (there are more studies on this with genital herpes as compared to oral herpes). For the vast majority of the time when no symptoms are present, the virus is dormant and not contagious. So, speaking in terms of probabilities, the virus was likely not present when the kissing occurred because there was no symptom at the time. But...
You very well could already have oral herpes and just not realize it. Each time you've kissed anyone - parent, friend, relative, partner, etc. - you've taken a risk for contracting oral herpes. Kissing is so common throughout childhood and adolescence that most people contract the virus then. Anywhere from 50% to 90% of the US population could have oral herpes and the chances of having it only increase with age. While most people have it, it's common for people to have it unknowingly.
So what to do now?? You could get tested (to see if you have either HSV-1 or HSV-2 make sure to request a "type-specific IgG blood test"). In terms of kissing, the best approach to reduce any risk of transmission is not to kiss someone when either of you has a symptom in or around the mouth. If a symptom is there and it's caused by herpes, it should heal on its own soon.
Finally, if you ever happen to notice a symptom of oral herpes now or in the future, please know that it may have nothing to do with this particular kiss. It could have been something you contracted a long time ago.
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