Riddick is pretty easy, it's much the same as Doom3. Bring down the console with the Ctrl+Alt+~ keys and type in timedemo [name of demo]
and hit ENTER, at the end of the benchmark bring the console down again to view the results.
ps, Riddick doesn't come with Demo that you can run but if you patch the game with the v1.1 patch it installs 5 demos for you to try, they are called....
so you would type into the console timedemo panoptical01
(or any of the other names) to run the demo. You can also download a couple of demos from this SITE
called 44 & ducche