Make 2 groups of 4 marbles and weigh. The group that weighs more, split it into another 2 groups of 2 marbles and weigh. The group that weighs the most has the heavy marble, and to find which one, put one in either palm and feel which one is heavier...?
I don't really know...
P.S. If you're wondering why there are two answers the same, I forgot to login, so..