How to check whether my Intel-based Mac is 32-bit or 64-bit
To tell if you are running the 32-bit or 64-bit kernel (which matters for some device drivers), launch System Profiler and click on the Software heading in the Contents section.
The line 64-bit Kernel and Extensions will say Yes if you are running the 64-bit kernel and No if you are running the 32-bit kernel.
Note: Mac OS X 10.6 Snow Leopard is the last release that a 32-bit Intel Mac can run, Mac OS X 10.7 Lion and up are, by default, 64-bit. Additionally, System Profiler was renamed to System Information in OS X 10.7.
Run this on the command line:
getconf LONG_BIT