you can tell very easy on your own. bottlenecks vary program to program.
so, fire up whatever game it is you are playing. make sure task manager is running so you can see your cpu usage. play a little, and then see how active your cpu cores were. if they were running at 100 percent, you have a bottleneck
its also entirely possible whatever you are running doesnt support sli
and as the gent above me said, overclock that cpu. its begging for it