Mirjam, you can preview the effect your filter settings will have by double clicking on the clip you want to affect. Click the filter tab in the *Viewer* pane, adjust the sliders and you will see the changes in the *Canvas* pane. The playhead in the timeline must fall within the clip you want to preview. If it is not, simply slide it to the clip you’re working on and cast your glance at the Canvas.
It would be mind-bogglingly difficult to apply filters if you couldn’t see what you were doing!
Good luck with your project, Matth