On Windows 11 (build 22000 and later) you can call GetMachineTypeAttributes API with appropriate image machine constant. If the function succeeds, it tells you in which modes such code can run. For applications (user mode) we are interested in the lowest bit.

Interesting bit: In latest versions of Windows the ARMNT (32-bit ARM) is no longer available. Future ARM CPUs are dropping it, so is Windows.

On older Windows than Windows 11, all non-native architectures run under WoW64, so you can check using IsWow64GuestMachineSupported API.