Go to your apple menu at the top left of your scren and choose About This Mac. When the dialogue box appears choose More Info. After you choose more info you will see Apple System Profiler at the top of the screen. Click the little triangle beside Memory Overview so the triangle points down. Listed there it will tell you how many RAM modules you have installed and how large they are. If they are recognized consider then working unless you are experiencing crashes, odd behavior, etc.
As an aside you can click the other triangles to face down to check what else is in your machine. Click the tabs at the top to see hard drives, applications etc. Good luck.