Mac OS X: How can I tell if I'm in 64-bit mode?

Go into System Profiler and select Software from the left pane. In the right pane you will see 64-bit Kernel and Extensions. If it says yes, you are in 64-bit mode, if not, you're in 32-bit mode.

"For 10.6, John T's answer is right. Just for whoever gets here with the same question for 10.5: find the program Chess in Finder, right-click, Get Info. You'll see an option to fall back to 32 bit, if 10.5 is capable of running in 64 bit mode:



(This also works for non-Intel hardware.)"