A quick fix while in a situation where you want to "test" a part that you need to stage to in order to test it, is to create a new stage and place the part into it, then move the stage to the end of the staging list (make it the next stage) and stage to it. This will work if the part has already been staged to, such as an engine you may have used to get into orbit, but must stage to while in orbit. This is probably an exploit that they will fix, or perhaps they will put manual test buttons on all parts at a later date.
