A very useful and somehow little known feature of Flash is the option to 'clone' the stage and have as many instances of it as we need.
This allows us to work inside a symbol and see how it affects the whole scene.
Also we can look at multiple frames at the same time – for example # 7 in one window and #63 in the second. In the old versions of Flash the timeline could snap (dock) to the stage windows, so we could also see multiple timelines at the same time too. It seems that this is not possible any more.
To open a second instance of the stage we need to simply choose Window > Duplicate Window from the menu bar and then undock and arrange the two so that we can see what we need.
Another little known feature is the Full Preview mode. It can be activated from the View menu by selecting View > Preview Mode > Full.
No matter how many levels deep inside the symbols we are, selecting Full Preview Mode shows all the content in its real colors. This in combination with View > Hide Edges allows us to design and choose color in context without the typical reduced contrast of everything that is not included in the currently edited symbol.
When this Full Preview Mode is no longer needed we can go back to the default by choosing View > Preview Mode > Anti-Alias.