Assuming you know what the new memory amount should be (existing memory plus new memory amount), your operating system has an option to display total memory available.

Windows
Right mouse-click on the "My Computer" icon and select "Properties." The total memory is displayed under the "General" tab.

Mac
Click "About This Mac" or "About This Computer" from the Apple menu or Apple symbol in the top left corner of your Desktop. Total memory amount should be displayed.