There are a few programs that could tell u how many slots ur using. One such one is CPU-z. It would give u quite a lot of info, but check the 'Memory' tab when running it and it should tell u how many RAM slots are being used.http://www.cpuid.com/cpuz.php
The download link around top left should do it
Installing RAM is also pretty easy, provided that u have easy access to ur empty RAM slot (when i say easy...i mean u dont have to take the keyboard off or something to get to it. On mine i just have to take off one screw and remove one flap on the bottom of my system.). Just google for a guide thatll instruct u how to install RAM, since u have to slide it in at an angle and then press it down (gently :P). Remember to keep urself grounded...or u might end up damaging ur system.