Astro is an excellent app. You can also mount your card to your computer and review the contents of your card that way too. (plug usb cable from phone to computer, from phone's notification pull-down-bar mount usb)
lefty78, you're looking at your SD card from within the Android OS. It's the same as if you were looking at it mounted to your PC/Mac/Linux box. What in particular are you looking for? Maybe I can point you at the specific folder.
When you open using astro; your most likely looking at your sd card and looking at what's on there which are sub folders (doorways) to your stuff stored on your sd card; your music, pics, videos etc. But you may need to say what your looking for so someone can help you more on what your looking at. Just think of the sd card, like the hard drive on your computer.