Other than such effects with unmanaged code I believe it is largely irrelevant which framework an application is compiled for. Only extreme counterexamples come to mind, maybe you might want to compile an application to x64 that requires an insane amount of RAM. But the typical run-off-the-mill .net application will not care what processor and what mode it is run under. But if you WANT to find out, you can always check the size of IntPtr as suggested in the other answers.