Go to IIS Manager -> Application Pools -> select the app pool you want and -> Advanced Settings.
In there there’s a setting called “Enable 32-bit Applications”. If that’s true, that means the worker process is forced to run in 32-bit. If the setting is false, then the app pool is running in 64-bit mode.
Also you can open up Task Manager and check w3wp.exe. If it’s shown as w3wp*32.exe then it’s 32-bit.
More info here.