Mac : check if my intel based Mac is 32-bit or 64-bit

1. Choose About This Mac from the Apple () menu in the upper-left menu bar
2. See what Processor it has.
3. Compare your Processor Name to information below to determine whether your Mac has a 32-bit or 64-bit processor.

Processor Name	32- or 64-bit
Intel Core Solo	32 bit
Intel Core Duo	32 bit
Intel Core 2 Duo	64 bit
Intel Quad-Core Xeon	64 bit
Dual-Core Intel Xeon	64 bit
Quad-Core Intel Xeon	64 bit

Source: Extracted from http://support.apple.com/kb/ht3696