It's sometimes possible to tell that there are items above and below the visible ones when the item boundaries don't align with the top or bottom of the panel, but this depends on the height of the items. At minimum, there should be indicators that appear when there are offscreen items above and below the visible ones.
A scroll bar would be even better because it shows where in the list you are, and ideally the height of the moving part is based on the fraction of the list that's visible, giving a sense of how long the list is. It doesn't necessarily need to be dragable with the mouse, but that would also be nice.
Great idea. A scroll bar which you could move with the mouse would be an excellent feature. It would save leaning on PgUp and PgDn to navigate up and down really long lists.