While communication is necessary for you to understand their friendship and not to worry, it is also crucial to remember the conversation the next time they meet up and not bombard them with questions when they get home. Lack of trust becomes extremely irritating and can cost you the relationship. Its ok to be jealous at times, it shows you care for the person and are protective of them, but only in small doses is it cute and, to be blunt, acceptable.
Does this concern stem from a past relationship you had? Tough. Remember that everyone is different and whatever was done to you by your last boyfriend or girlfriend has no bearing in this new relationship. Just because he or she may have betrayed your trust, does not mean your new mate will do the same. They are different people with different opinions, which after an experience like that you need, right? Just remember, jealousy can be nice to show you care, but too much will cripple the relationship causing you to lose that thing you were so desperately trying to protect.