One of the best ways is to look at the HaveIBeenPwned service https://haveibeenpwned.com/ - Troy Hunt goes around spending effort finding these things, so whether he has found it is a reasonable indication of whether it is out there. There's a list of the breaches he has included at http://feeds.feedburner.com/HaveIBeenPwnedLatestBreaches

Edit - sorry, a better list of breaches is at https://haveibeenpwned.com/PwnedWebsites