If are not much aware of computer hardware or how to open the cabinet of CPU, you could use softwares like speccy from piriform or other utilities that will show you version being run whether DDR2 or DDR3 as below :

If you can open the cabinet of system then remove RAM from slots and check whether :
distance between left side of RAM and notch = 7.1 cm (2.8 in) then its DDR2 if its 5.4 cm then DDR3