How do I know if my 'My Passport for Mac' has backed up all my information?

I needed more space for my macbook air, so I bought an external hard drive to store all my stuff in there. I have the 'My Passport for Mac' device, and I think I synced everything up, but how do I check and make sure that all of my information is saved correctly to the external hard drive? I need to make sure before I delete all of my information from my mac. PLEASE HELP