(C#) Determining whether the current build mode is Debug or Release

I prefer the [Conditional("DEBUG")] attribute over #if DEBUG.
This is especially advised for your logging example.

E.g.

[System.Diagnostics.Conditional("DEBUG")]
private void Validate() { /* check instance integrity */ }

or

[System.Diagnostics.Conditional("DEBUG")]
public static void DebugLog(string message) { ... }

You call these methods like any other method. The compiler will not add any call code if "DEBUG" is not defined.

BTW: Your resoning of "not Debug" equals "Release" is not true in general, you might have other configurations than only the default "Debug" and "Release" configurations.