Welcome to the MacUpgrades.co.uk Mac Model Identifier tool. You can use these drop down's to find your precise Apple Macintosh, and see upgrade products which are 100% guaranteed compatible with it.
See the animated gif above for a guide on finding your Model Identifier - note that this video is for OSX 10.10 Yosemite, in some other versions of OSX you access System Profiler by clicking a button labled "More Info" rather than one labled System Report on the window that comes up after selecting About This Mac from the Apple menu. Once System Profiler comes up, the Model Identifier is listed in the Hardware Overview (displayed in the right-hand panel of System Profiler when it is opened)
You can also launch System Profiler directly, it is located in Applications>Utilities (that is inside the "Utilities" folder that is inside your "Applications" folder).