There is a call "IsDebuggerPresent()" which tells you if a debugger is attached.
(This will probably also return "true" if the program was compiled in release mode and a debugger is attached)

Also there are logging macros which are only compiled in debug mode.
So there must be a macro (preprocessor symbol) which is defined in debug mode only, but I do not know the name.
Possibly study the logging macros and their source to see how this works.

Alternatively you can add your own macro to the compiler options.