You can take a look at the version of the referenced assembly mscorlib. For Silverlight 4.0, it will be 2.0.5.0. Also, when you go to the class library project properties, you will see that it is Silverlight. Visual Studio also uses various icons to represent them:

, .csproj, , Silverlight GUID, Visual Studio :
<ProjectTypeGuids>{A1591282-1198-4647-A2B1-27E5FF5F6F3B};{fae04ec0-301f-11d3-bf4b-00c04f79efbc}</ProjectTypeGuids>