How can I find out in a 32-bit application what 64-bit processes are launched?

Documented capabilities of WinAPI do not allow you to directly identify 64-bit processes from within a 32-bit program launched in a 64-bit Windows under WoW64. In general, reading of memory of a 64-bit process from a 32-bit application is impossible due to the difference in address space arrangement.
You may get a list of 64-bit processes for a 32-bit application either by using a specially created 64-bit process you may exchange messages with or using an out-of-process COM server (for instance, DLL Surrogate).

References

1. Discussion at Stackoverflow.com. How to enum modules in a 64bit process from a 32bit WOW process.
2. MSDN Library. EnumProcessModulesEx Function.