One of the most prevelant mode of infection is headed through malicious web-sites. An attacker may chose to host his own site & use it for malware propagation. Or better still, exploit vulnerabilties in other web site(s) & host his malware code on them.
There are different ways to verify if a site is hosting malicous script(s) - varying from tedious manual inspection of code, to using tools such as Malzilla (http://malzilla.sourceforge.net/) & several others, or verifying with Google Safe Browsing.
wepawet is a simple service/tool that many security professionals prefer to use for analyzing web based malicious code.
wepawet analyzes a web-site & provides us with good information about the malicious resource, & the vulnerabilities exploited during an attack.
Basically, wepawet is a simple point, click & shoot tool for analyzing web-based malware.
You can check out wepawet here: