Take Advantage of Transparency Grid in Adobe Illustrator
Show Transparency Grid is command in Illustrator's View menu and can be quite useful to check which areas in artwork contains fill and which are transparent or without any fill. Beside View menu you can use keyboard shortcut Shift + Control + D to show/hide trasnsparency grid.
|Show/Hide Transparency Grid from View menu|
Product used in this tutorial: Adobe Illustrator CS5