How to detect which .NET runtime is being used (MS vs. Mono)?

I would like to know during execution of a program whether it is being executed using the Mono runtime or the Microsoft runtime.

I'm currently using the following code to determine whether I'm on a MS CLR:

static bool IsMicrosoftCLR()
{
    return RuntimeEnvironment.GetRuntimeDirectory().Contains("Microsoft");
}

However, this is somewhat dependent on the installation folder of the runtime and I'm not sure whether this will work on all installations.

Is there a better way to check for the current runtime?