Jul 12 How to Check if Your Yahoo, Gmail, MSN, Hotmail, Comcast and AOL or Other Passwords Were Recently Stolen and Published (2012)

FIRST read this article from the reputable website Mashable to learn how 400,000+ Yahoo, Gmail, MSN, Hotmail, Comcast and AOL [and other] email accounts and passwords were stolen and published, making them vulnerable to identify theft.
THEN go to this link in that article [this is safe; i did it] to see if your email login and password were compromised by the July 11 hack into Yahoo.com. [you enter only your email address, not your password.]
This is just one of ever more frequent thefts and publication of large numbers of email accounts and passwords. 
THEN even if your email account/password was not listed as part of the theft from Yahoo, it would be very prudent to change your passwords now and often in future. 
See also