Depending on which graphics engine the game was built, you can check how many cores it utilizes if you search at the files and folders of the game you have installed at your system.
For example Tera, based on UE3, at the installation folder has a file named S1Engine.ini. You can use a notepad to open it and tweak it and see how many cores can utilize.
Attention: Before any tweak, back up the file, after your tweaks (for example if it uses only 1 core, you can change it to 4, if you have a quad core CPU or even add a line like "CPUNumLogicalProcessors=4" at the very end of the S1Engine.ini) change the file properties to "Read Only" so the client during the lunching of the game doesn't revert back the settings.
The above is a general guide, and you can always use Google, for a specific game of your interest.
Most games though, i have the suspicion utilize 1-2 cores.