In the tools menu, there's an option for timedemo. Just select it and the fps and avg. fps is displayed on the right hand side of the screen.
Some people run it on the opening sequence, but I don't like this since it doesn't have any actioin in it. I prefer to record my own demo and use it as a standard for making changes to my .ini file.
for a demo hit the tilde key "`" and use the following commands:
STOPDEMO-stops recording or playing a demo
DEMOPLAY utfilm - plays a demo, in this case "utfilm"
DEMOREC utfilm -records a demo, "utfilm"
If I remember correctly, you can leave off the demo name and it will record a demo named "utfilm" automatically..but next time you record, it will also name it "utfilm" and record over the one that's there.
Using your own demo works good if you need a reference for modifying your .ini file,but of course it's no good for comparing your mac's performance with other macs.