How do I tell my code is running on Mono?

From the Mono FAQ:

http://www.mono-project.com/FAQ:_Technical

Below is directly from that link:

How can I detect if am running in Mono?

Having code that depends on the underlying runtime is considered to be bad coding style, but sometimes such code is necessary to work around runtime bugs. The supported way of detecting Mono is:

using System;

class Program {
    static void Main ()
    {
        Type t = Type.GetType ("Mono.Runtime");
        if (t != null)
             Console.WriteLine ("You are running with the Mono VM");
        else
             Console.WriteLine ("You are running something else");
    }
}

Any other hack, such as checking the underlying type of System.Int32 or of other corlib types, is doomed to fail in the future.

Long and short of it, just don't.