Why I reccomended this site to check..? Because it's more accurate than another (http://www.yahooscan.net/, http://ymdetector.net/). The two website cannot show my status Invisible when I'm Invisible. They Show Offline.
Wanna try..? just visit http://www.invisible-scanner.com/ then insert Yahoo Messenger ID that you want to check.
You can get the result after click the CHECK button.