If your site is acting funny or if you see something you don't normally see, there may be a problem.
Sometimes, even when you take precautions, a site can be vulnerable. Let's face it, if major institutions can be hacked, anyone can. iThemes Media LLC who provides an excellent security plugin has released an equally excellent article and a handy infographic to guide you through some potential issues.
If you're concerned at all that something doesn't look quite right on your website, visit the article "Is My WordPress Site Hacked? 7 Signs of Infection."