A trail ride on a strange horse can turn out quite nice if you take a few precautions before you go:

Ask the owner of the horse about the horse's character, behavior patterns, training issues etc
Wear a helmet. It is a wise practice to wear a helmet when you ride any horse. However, people very often do not use helmets on horses they know well, or on quiet trail rides. To wear a helmet on a strange horse must be a habit.
Do not go alone. Have someone experienced with you who rides a well-broken and well-known horse..
You must always pay attention to a horse you are riding even if it is your own. However, on a strange horse it is many times more important. At the end of the ride you will get a bonus. You will know the horse much better.
Be prepared for an unusual behavior and reactions. Do not assume that the horse will behave as other horse you rode on a trail ride last time. Do not get mad at the horse or punish him/her if you don't understand why the horse did something.
If you are not a very confident and secure rider do not try things that can go out of hand. For example, canter in the open field. You may get reactions that you are not prepared to handle such as bucks, jumps or bolts.