I usually perform ghusl after taking a shower, meaning all my body and head is wet before the ghusl. While performing ghusl I can't be 100% sure if the part I'm washing got wet with the water of ghusl or because it was wet before. What should I do in this case if i can't be completely sure whether any part of my body or head remained unwashed during ghusl?

There is no harm making wet your body before having the Niyyat of Ghusl. Normally when you wash head and Neck and then right side of the body and then left side. Your Ghusl is in order no need to having doubts unnecessarily.