Detecting ASP.NET

I've been wondering for a while how you could reliably tell if you are currently running under ASP.NET. This is really only of interest to be because of the ThreadSingleton vs. Static Singleton issue. The best way I've found so far is:

bool isASP_NET = ( System.Web.HttpContext.Current == null )?false:true;

Kind of annoying, because you have to reference System.Web in your Project, which you wouldn't otherwise.

I looked through most of classes that come out of the System and mscorlib assemblies but couldn't find anything good and reliable (i.e. didn't want to use AppDomain and see if the config file was called web.config.. Sounds like an accident just waiting to happen.) Still, there's got to be a better way.