There are so many free, freemium and premium Themes out there. How can I be sure that a Theme I download doesn't have malware in the code? Is there an (relatively) easy way to check for malicious code without going through every line of code?
You should try using the theme check plugin at http://wordpress.org/extend/plugins/theme-check/
Antivirus Check for Wordpress themes: http://wpantivirus.com
Lot's of real life detection for malicious injections in the website files: http://quttera.blogspot.co.il/search/label/Web%20malware%20scan%20reports%20and%20analysis
Look for similar stuff in your website files.
To do the whole sitecheck from your WP dashboard for exploits and malware use this one: http://wordpress.org/extend/plugins/quttera-web-malware-scanner/