Simple question, simple answer; if you haven't got one, leave it well alone. An alternative simple answer: if it can't be done fully clothed don't do it. Simple answer number three: if you wouldn't do it in front of a small child, don't do it until you are married.
More complex answer: avoid any sexual contact at all. Holding hands is not always sexual since parents and children do it and so do christians during certain songs in certain churches.
Kissing (mouth CLOSED) is not always sexual: in many cultures, including the Biblical culture, kissing even on the mouth was done routinely between friends and relatives. Old ladies (at least over here) expect to be kissed on the mouth by young friends and relatives, even today.
Exploring each other's tonsils is foreplay, plain and simple, and so is breast fondling. Deep kissing can be simply replusive if it leads nowhere and breast fondling can even be painful if done by a young overeager man. I speak as one who pulled out of marrying a man who was (and still is) a very close friend, simply because the feeling of pressure caused by this sort of thing (which is nothing by the standards of most!) was intolerable. With the man I eventually married, we did nothing, and I mean, absoltuely nothing, beyond hugging, hand holding and a chaste closed mouth goodnight kiss, until we were married. I cannot tell you how much better that is, from a woman's point of view, than letting the raging hormones dictate what you do.
Liz