bitly uses data from a number of independent sources in addition to its own internal classifiers to determine whether or not destination sites propagate spam, viruses, or other malware. The third party sources include Sophos, Websense, VeriSign, PhishTank, and Google Safe Browsing.
If you aren't so sure about where a bitly link will direct to, it is possible to preview the bitly link before clicking on it.
To do this, simply add a + sign to the end of any shortlink in your browser. For example, for http://bit.ly/Wn2Xdz just enter http://bit.ly/Wn2Xdz+ into your browser and you'll be sent to a preview page for the link.