I would recommend you go to Dells website to obtain the manual for your specific system.
Here's the link: http://support.dell.com/support/topics/global.aspx/support/my_systems_info/manuals?c=us&cs=04&l=en&s=bsd&
Click on the choice to "Choose by service tag" and enter your service tag. Then download the manual for it. In the manual will be a section about diagnostics and diagnostic lights.