6 feet is a good height, but if they are really determined, they can fly up to it. You just have to make them not want to go to the other side. Definitely clip the flight feathers off one side. As they get older and heavier, they are less likely to go over. Is there a Rooster in the group? I find that roosters tend to keep the girls close by. My fence is only about 5.5 feet and when they were young, a few of the more flighty birds would go over (looking at you leghorns!), but at about 1 year, they were too heavy (except the leghorns) to get up that high.
With my birds, if one got over the fence, they would then spend the next hour worrying how to get back to the inside. Chickens are cute, personable, but not always the brightest..
They are more likely to exploit any gaps along the bottom fence line, so double check and make sure it is tightened up.