When dealing with an unfamiliar website / web business, I use these tools to research the site's reputation and scan for malware.
Reputation Checkers
These checks are quick--they check the URL against blacklist databases and URL reporting sources. Enter the URL and get the rating.
BrightCloud
http://www.brightcloud.com/tools/url-ip-lookup.php
Cisco Senderbase
https://www.senderbase.org/
FortiGuard
https://www.fortiguard.com/static/webfiltering.html
McAfee TrustedSource
http://www.trustedsource.org/
Norton Safe Web
https://safeweb.norton.com/
TrendMicro Site Safety Center
http://global.sitesafety.trendmicro.com/
WatchGuard Reputation Authority
http://www.reputationauthority.org/
Web Of Trust
http://www.mywot.com/en/download
Webutation
This one combines reports from several reputation-checking sources.
http://www.webutation.net/
URL Scanners
These checks are more comprehensive, but take longer because they actively scan the site for malware. Enter the URL, and it will be scanned.
Comodo Site Scanner
https://cwatch.comodo.com/sitescanner
Quttera
http://quttera.com/
Sucuri
https://sitecheck.sucuri.net/
VirusTotal
https://www.virustotal.com/en/