On Windows, select Start -> Control Panel -> System and check whether System Type says 64-bit or 32-bit.
On MacOS, go the Apple menu, select About This Mac -> More Info -> System Report -> Hardware and then look at the Processor Name field.
If it says Intel Core Solo or Intel Core Duo, you have a 32-bit machine.
If it says anything else (including Intel Core 2 Duo) - you have a 64-bit machine.
On Linux, open a Terminal and run the command uname -m
A response of i686 means 32-bit, and x86_64 means 64-bit.