How do I scan for hardware changes? I have had the times where I have used a usb networking card, or other device, and it didn't show up, although I was not sure if it was compatible with OSX. I just was not sure if OSX had a way to scan incase it did not recognize it, or if you needed to install a driver.

Is there a way to see what hardware is on the machine, like a device manager?
Generally speaking there's nothing you have to do except launch the application that supports your hardware.

If you add a new network device, it is immediately listed the next time you open the Network preference pane in System Preferences.

If you add a new sound device, is it immediately listed in the Sound preference pane in System Preferences. (You can also Option+Click the volume icon in your menu bar within seconds of plugging your device in and see it in the list.)

You don't need to "add new hardware" to use it. If it is compatible, it generally just shows up.